I think the biggest tragedy of the Bruce/Natasha ship is how the MCU lined it up so well, but missed every pin.
The characters had a lot in common, and a lot of potential to develop together. They both saw themselves as monsters. They both wanted to recompense for the damage they'd done. They were both unsure where they fit into the world.
They even had a small arc in the first Avengers film. Natasha initially feared Bruce and Hulk, then, the helicarrier incident occurred and Hulk attacked her—yet near the end of the attack, he hesitated to kill her. This could be construed as part of the reason why, at the end of the film, she readily encouraged Bruce. And while at the beginning of the film, she slid a phone across the table to avoid handing it to Bruce, after the battle had ended, she handed him a bag.
That ending created the foundation for a romantic relationship, simply by virtue of their progression from nervous colleagues to comfortable allies, which could have easily progressed further. The hint at a future romance was present in their dialogue in the first film too, and so was the chemistry. Natasha remarked that Bruce "didn't come because she batted her eyelashes at him". Natasha said she would "persuade him", and Bruce looked thirsty. Natasha looked at Bruce with pity when Loki insulted him in the containment cell, and when Bruce noticed this, he smiled, be it in amusement or something else. The film didn't need to show us that small interaction, but it did.
AOU was released, and it tried to continue this. Unfortunately, it showed us a relationship that failed to take advantage of any of the foundations established in the first film. There was no "connective tissue" between the two films to sell their dynamic. Nothing that happened in the first film was mentioned and built off. The only reason Natasha gave for her interest was that "their teammates were fighters, but unlike them, Bruce ran from the fight even though he knew he'd win". It was a surface-level observation that anyone could have made, not tailored to her character.
In other words, Natasha was being molded to fit events, rather than events being moulded to fit around what she would do based on story logic.
Their overall dynamic was odd. At the beginning, there was a dissonance where Bruce was unaware of Natasha's courtship—this was clear in his dialogue with Steve at the party, where he admitted he had no idea she was flirting with him—yet, the Hulk was somehow receptive to her. This was never explained. Was it because Bruce repressed himself so much that he thought he didn't see anything, but his sentiments surfaced through Hulk? If so, the film gave no signs to pick up on, not without excessive extrapolation that veered into Fan Theories territory. Perhaps the dynamic was an extension of how, in the first film, Hulk hesitated to end Natasha. If so, that's a very small thing, and like everything else, it wasn't carried forward or mentioned.
(Unfortunately, Bruce never got an on-screen vision from Wanda. I think this could have been a good time to explain some of his behaviour, and Hulk's.)
The conversation between Bruce and Steve at the party, in a way, was ambiguous enough that it could've been viewed through a lens where Bruce knew Natasha liked him. In that case, his dialogue wouldn't have indicated obliviousness, and instead, an attempt to throw Steve off his scent. This would've been supported by Steve's remark about breaking by-laws. However, this simply wasn't verifiable from how the scene was written. Based on the dialogue, especially the dismissive lines like "She flirts with everyone", Bruce could just as easily have been oblivious as cognizant, especially because the film characterized him as timid and awkward, which would've watered down any visible romantic interest in Natasha he showed. If the film wanted to go that way, especially with how Bruce was characterized, there needed to be less ambiguity, not more. Bruce and Natasha also shared screen time with a large cast of characters, so when they did appear, their beats needed to hit clearly so their relationship sold well and didn't get lost.
Then came the scene at the homestead. At this point, Bruce seemed to know he had feelings for Natasha, because he didn't reject her advances right away. Only when Natasha got too close did he back away. Feelings of that extent, for Bruce, hadn't been conveyed in the film prior to this. It felt like he'd been at a 1/10, then after Johannesburg, he ramped up to a 6/10. It's unclear what caused Bruce to fall for her, between the events of the party and the homestead scene, as there was no defining moment in the narrative. It was murky. Perhaps Bruce took Steve's advice of "Don't wait too long" from the party to heart, but again, he hadn't shown any interest in her at all up to that point, and he didn't act on Steve's advice in a manner that was specifically cordial to Natasha. He did gesture for Natasha to try lifting Mjolnir, but other people verbally invited her at the same time ("Widow?").
Bruce's decision to run away with Natasha, even accounting for his earlier trauma with Hulk, didn't make sense either. It relied on Bruce having preexisting feelings for Natasha which, again, he hadn't readily demonstrated in the text. If he had, it didn't have time to marinate in an apparent enough way.
This also caused his comment about "having a compelling reason not to lose his cool" to land emptily.
Later in the film, at the hideout, Natasha forced Bruce to Hulk out. I didn't mind this decision, at least in a vacuum. Natasha's training, and her mental math of the situation, probably justified betraying Bruce in order to save everyone. However, there was a disconnect—she didn't seem too broken up about what she did. If she had fallen for Bruce, she should've shown it. This didn't need to be an emotional breakdown, but just something that showed she was affected.
Natasha was a hardened assassin. Maybe she wouldn't have flinched at the idea of using him. However, the film had just spent an hour showing Natasha wearing her feelings on her sleeve for Bruce. It had established that she could express herself when it came to Bruce. People have criticized this readiness of expression as being out-of-character for her, and perhaps that was true, but unfortunately, it was canon here, and the film had to abide by that. Natasha's reaction at the hideout didn't fit. The film broke its own canon in order to bring Hulk back.
Not only did this muddle that particular scene, but it cast doubt on the previous scenes and the quality and consistency of their writing.
The fandom's reaction to the relationship was overwhelmingly negative on release. Many fans said Bruce and Natasha lacked chemistry. I have to agree, because Bruce showed little solid interest in Natasha despite her courting and Hulk's receptiveness. Audiences don't need to be spoon-fed, but they don't need to starve, either. There was a balance that needed to be struck, and I'm not convinced it was struck.
People also claimed their relationship in general came out of nowhere. I don't agree with this claim. As explained earlier, the underpinnings for a romance were present in the first film. If two characters receive ample screen time together, and the trajectory of their relationship continues to improve until the end of the film, it's reasonable to assume that it could progress to romance. However, the execution of the relationship in AOU was confusing. The two characters felt like Barbie dolls being smashed together.
The infamous line about Natasha's sterility didn't help this case. At the homestead, she explained that she, like Bruce, couldn't bear children because she had been sterilized. Then she asked him, "You still think you're the only monster on the team?". The question directly followed her explanation about her sterility, so it conveyed, through context, that her being a monster had resulted from it. The intent had probably been for Natasha to compare her own "mindless violence" track record to Bruce's, but the mention of sterilization, and at the time she mentioned it, came off as clumsy and offensive.
In my mind, it would've been better to make Bruce and Natasha enter a relationship on-screen, with both of them knowing what was going on, while peppering in connections that built off what happened in the first movie. We still could've gotten oblivious Bruce at the beginning, and it would've explained Bruce, Hulk, and Natasha's dynamic in a way that didn't throw the audience into the deep end without a life jacket. Yes, it would've meant removing Natasha's role in the lullaby, but in exchange for making their relationship clearer and less ambiguous (in a bad way), which for storytelling is a no-brainer. Alternatively, the film could've opened with them being in a relationship, using the lullaby normally, and presenting references (bold or subtle) to the previous film to explain the changed dynamic.
My last issue with the film version of Bruce/Natasha is more of a meta commentary on fandom. Since canon dropped the ball, fans who had a preexisting bias against Bruce/Natasha viewed canon as "proof" that it simply didn't work, period. Due to this, they saw no difference between canon and any other versions, and it spurred even more aggressive hate and hostility toward shippers than usual. People shouldn't be bullied over their ships, ever, but it's especially disappointing when people are being targeted for something they are only assumed to like.
Countless popular ships aren't canon. Suffice to say, fans can still have their own interpretations even if canon drops the ball for them, because canon isn't the be-all, end-all. Otherwise, why do transformative works exist? Or shipping? Or fandom in general?
In 2012, I remember going into the Brutasha tag and finding it empty. I really wanted the ship to become canon. But canon could've done a lot better with it. The fandom could've treated the people who did like and ship it better, too, no matter what flavour of Brutasha they preferred. It's kind of a shame, because I truly do love this ship.
