For one in my opinion the built in pathfinding is made for an RTS type game, its especially bad for grid/board based games
You can use pathfinding without the movement, and just get it to return a list of the nodes in the found path and implement your own movement or use for the path. You don't have to use its own movement. So doesn't that help it cover other kinds of games?
The behaviors are one of the banes of C2's existence. They are wonky and buggy and after years of work just don't cut it, even for simple retro platformers or zelda-likes
Behaviors are intended to help beginners get started quickly, and also provide shortcuts. For example the bullet behavior is pretty convenient if all you need is something moving in a straight line, saving you having to set up the events to move it with x += cos(angle) * speed; y += sin(angle) * speed etc. They're not intended to cover every possible genre of game - how would that even be possible through the behavior system? Only events can implement custom logic, or custom Javascript coding with the SDK. I think hoping for behaviors to cut it for everything is hoping for too much from them.
Modularity is definitely an interesting area to explore in future, but I can tell you the platform behavior has some very complex and subtle logic to cover a couple of year's of edge cases reported via bugs. I think one of the risks of event-based behaviors is that it's actually pretty difficult to write a flexible, general purpose movement without just fragmenting it in to a bunch of entirely different behaviors. Then the question is which do you choose, why, and can you change your mind later?