Sparity, as it is in canon, I'm not super into due to these reasons:
It's Adult x Minor. Nuff said.
The “romantic” side of their relationship is strictly one-sided, and therefore boring.
Spike stalking Rarity and sleeping with a Rarity doll is not cute, it's creepy.
Rarity remaining oblivious to Spike's obvious feelings for as long as she did makes her look like an idiot.
If she's 100% aware of his feelings for her, it makes her look like a giant creepo. Because instead of gently letting him down like a responsible adult, she allowed him to continue on with it (possibly in order to manipulate him into continuing favors for her). Also it makes her behavior in “Dragon Dropped” look even creepier…
However, if we were to do what we normally do and completely ignore canon, I can actually see Sparity working.
First of all, Spike has to be an adult. Nuff said!
Think about it: Rarity is the Element of Generosity. Dragons are known for being greedy. They literally have a magical biological response to greed, where they'll turn into mindless giant monsters the greedier they get. Why wouldn't Spike admire Rarity's personality, since she inspires him to reject his biological instinct to be greedy, as shown in “Secret of My Excess”.
Rarity and Spike both love crystals and gems. It's shown in “Dragon Dropped" that Spike tastes gems for Rarity to determine if they're high quality for her to use/buy.
In the same way Rarity helps Spike keep his greed in check, Slike helps Rarity from going overboard with her ideas by being real with her which is what he learns in “Inspiration Manifestation”.
My Little Pony is infinitely better when you ignore the dumb/problematic things in canon, ain't it. You can say that about any franchise actually. I genuinely believe that if Spike were an adult and not a child, the writers honestly could've evolved their relationship to something romantic.
But it was never gonna happen while he was a child, because that's creepy as all hell. But ignoring the weird aspects, I can see Sparity working very well.