Alright, a lot going on there, I'll start with some info on me relating to Spider-Man. I've been a magic player for decades at this point. I have read Spider-Man comics borrowed from my local library plenty of time growing up, I wouldn't consider myself a Spider-Man or Marvel fan or anything, but I came into this knowing likely a bit more than the average magic player about Spidey, and less than the average spider-man afficionado.
I'll start with what I liked most: the special treatments! Most of the special treatments for the set were great and did an incredible job paying homage to comics. In particular, the paneled Sagas are the high point of the set for me, bridging together mechanics, art, and what makes comics comics all in one.
The Source Material cards aren't universally great, but out of all the Source Material bonus sheets we've seen before or since, this one works the best for me, particularly using the comic book covers already intended for print and being readable at a glance on shelves, it translates better to magic cards than most.
Mechanically, Mayhem is a nice and reusable simplification of Madness. It feels like a cleaner design of it. Villains Conniving was also a neat touch. There are a lot of interesting designs, particularly at higher rarity.
Through the Omenpaths' arts and reflavoring is pretty neat and does a good job into translating designs into their Multiversal counterparts overall. There are a few vestigial weirdness like the Hero or Villain types that feel arbitrary (particularly annoying in limited if one tries to build around either type on Arena,) but other than that it makes it feel at home (if very spidery) into Magic.
It also allows me to pivot neatly into what I didn't like about the Spider-Man set.
Through the Omenpaths existing is BAD. Bad enough I'd prefer not having a set at all if the alternative is doing this (though I wouldn't prefer having the set only in paper, which I assume is why this choice was made.) As someone who plays both in paper and on Arena, having two different set of cards with different arts and names be functionally the same is bad enough for a handful of cards like a secret lair. It should be disqualifying for a set with hundreds of cards. It makes talking about the set and following talk about it almost impossible. Not only when it released but with even more confusions now, months later. I look back and I see a bunch of cards I do not recognize, both in paper and arena.
It makes it worse that future Marvel sets are already announced and more of this can be expected for them.
It being a small set brought up painfully to a small-but-not-tiny size partway through hurts it as well. The limited wasn't great, but I'm only an occasional limited player so I'll leave people more experienced talk on that. But it was felt even in constructed. As someone who plays a fair amount of standard, it didn't have enough meat to really build any deck around the set themes. Web-slinging has maybe ten cards in the entire main set, most of which are aimed towards limited. Mayhem had barely more. We have a dedicated self-discard deck in current Standard in Rakdos and it typically doesn't even use any Mayhem cards. Villains had some typal synergy, but not enough for a constructed deck. Not even necessarily a tier 1 deck, just one that could compete. Maybe spiders?
Maybe spiders, indeed, but then again, that was also an issue. Spider-Man is the central character of Spider-Man, yes, but there is such a concept as too much of a good thing. Just in the main set, over 20 cards in the 200 card set start their name with "Spider-", and there are 40 if we expand beyond just the first word. One card in five. It makes both discussing and remembering the set much harder. This only makes the Through the Omenpaths issue worse. I know Kavaero is really strong in Standard and beyond, I think he's a Spider-Man in paper. But which one? Something like "S... Spider-Man" I'm pretty sure. Is it Spectacular? Supreme? Sensational?
Flavor-wise, I'm not the staunchest opponent of Universes Beyond, but some of the more mundane elements of the set do feel a bit out of place in this fantasy card game. This isn't a problem unique to Universes Beyond, I've had issues with it in other sets before, but some notable offenders from this set range from the Passenger Ferry to Rent is Due.
Mechanically and as far as individual designs, the Soul Stone feels pretty egregious at Mythic. It was stated by WotC designers before that purposefully printing generic staples for commander made the format worse, with Arcane Signet being paraded as an example. Arcane Signet was a common. The Soul Stone doesn't go in every deck of every color, but it seems pretty purposefully aimed at the same area of design, and it has much more limited distribution as a mythic that is unlikely to see nearly as widespread reprints in the future. Just like Through the Omenpaths, it is made worse by knowing it is likely part of a tight cycle with other upcoming Infinity Stones in future Marvel sets.
More of a peeve than an actual complaint, but for the sake of mentioning everything I remember: Spectacular Spider-Man not appearing in non-borderless version in the main set unlike literally every other card is weird and I would rather have a normal version available.