Fusion is a 29 page PBtA system that is strongly influenced by Steven Universe. It doesn't do much to obfuscate this, and honestly that works in its favor. It knows what it wants to be about, and it doesn't waste energy on making changes just for changes' sake.
Fusion starts with a structural overview of how PBtA works (even getting into the difference between a montage and a vignette,) and this might make it a good intro to PBtA for folks who haven't played anything on that engine before. At the same time, it might not be an ideal game for folks who haven't played any tabletop at all, as it breezes past explaining what the narrator is and only covers it much later.
The book's organization overall felt a little odd to me. You start with an overview and some philosophy, then you go into a breakdown of scene/session structure, then right into character creation, all the way through conditions and marking experience, then the dice are explained, then some bonus rules about fusion, and then an overview of the setting. The fundamentals feel a bit buried, although if you're already familiar with PBtA you should be completely fine.
Writing and design-wise, Fusion is great. Game elements are explained clearly. Game terms are super flavorful. Your stats are Cut, Clarity, Facet, and Luster, which feels evocative and neat. You also have Sync, a relationship system that blithely forsakes PBtA's preference for small numbers and goes all the way up to 12. There's also the titular Fusion, which lets you combine characters with someone else.
For something with a lot of potential to go wrong, the Fusion mechanics end up being a super strong piece of design, and they tie everything else together.
Every Fusion is a conversation, so no player loses agency. However, Fusions are only as capable of holding together as the component PCs' Sync. Furthermore, PCs in a Fusion can try to grab the reins, losing Sync but momentarily controlling the Fusion without having to ask.
The advantage of being Fused is that you roll 3d6 on Moves instead of 2d6, and this makes Fusion potent but high-risk. Meaning when it gets broken out, that's probably the culmination of a session, and it's going to end on a triumphant note---or a catastrophic one.
Overall, I think Fusion is a solid PBtA game that both appeals very directly to Steven Universe fans, and which has mechanics that would port very easily to a Pacific Rim or Evangelion game.
The Fusion and Sync rules, in particular, are a standout, and if you like any of the above franchises, you may want to grab a copy of this for the Fusion mechanics.
Minor Issues:
-Page 14, 2nd para, specifically talks about Steven Universe canon, which you might want to avoid if this game is inspired by but not actually Steven Universe.
-Page 15, tags, "a Fusion has the sum of mild and moderate tags of the component gems". Do Fusions start fresh, or are they filled if the component gems' tags are filled? Or does this mean just the damage carries over into the Fusion?