This might be a bit of a controversial take, but I found this game to just be kind of ok? IDK, I played the downloadable version but I don't imagine that's too different from the web browser build.
I'd say my biggest fault with the game is that it's too much reward without enough risk:
Enemies and bosses tend to be very slow and heavy. I think allowing bosses to get knocked around more, or at least try a lot more often to attack the player would help with this, and really sell your characters' strength.
The levels are mundane and largely uninteresting to traverse. I understand this is to make sure that enemies can be spawned and defeated often, but I think having level-specific enemy spawning or behavior would help to alleviate this issue and present more of a challenge to players.
Weirdly, I found the voice acting a little too intrusive: maybe some way of limiting its frequency would be helpful.
The crossover angle is done in a commendable, yet slightly disappointing way:
I feel that the playable roster played things way too safe by focusing almost exclusively on characters from FNF and Newgrounds Rumble: the kid from Dad n Me was a good start, but I feel like crossover games like this are at their best when they're breaking new ground with their character rosters (especially on Newgrounds, where we've seen Pico and co, Madness, and FNF crossovers out the ass for the past few years). I understand that this is done so that a more casual audience understands that its a Newgrounds crossover game, but it still makes for a roster that feels very "been there done that".
I did enjoy seeing a lot of games by some friends of mine get representation though: a lot of newer NG submissions tend to struggle for visibility, and it was cool. I hope more stuff like this is done with the game going forwards.