There isn't a strict polygon limit, Construct 2 will warn you if you create more points beyond 7 that you might face performance problems, but that's about it, really. As for assembling terrain, that's really not so much of a problem when tilemaps are thing. With tilemaps, you only need to set up tile collisions once for each tile, and for smaller tiles being part of a larger tile, you can also easily select an entire group in one go and place them anywhere on the tilemap, even flip and mirror it. It's really a lot more convenient, since there's a dedicated tilemap bar to easily and quickly grab what you need, rather than having to go into the object folders or the part outside the stage where you store the various tiles and copy-paste them. The actual curves aren't gonna be as smooth as you'd find with pixel-based collision, but I don't find it to be that huge a deal, and it could be worse, Stencyl and Unity are guilty of having problems with having 'hollow' collision polygons, that's not an issue for Construct 2.
My actual problems with trying to implement a Sonic engine in C2 stem mainly from optimization and some bizarre physics quirks, despite replicating Streak's code pretty much exactly. The engine used to run like shit, and that's just in a bare-bones level, but newer C2 optimizations with collisions have helped somewhat. I'll probably look over my latest capx with the latest beta and see how it holds up, and see if I can figure out where the bottlenecks are, but I think it's either collisions or the way the functions are implemented.
Edit: yeah, while the engine runs fine on my machine (though it is an Inside Core i7), it probably needs some serious optimization, especially if anyone even thinks about implementing multiplayer. That being said, it's worst problems are some oddities with the physics in specific circumstances.
This post has been edited by Candescence: 15 December 2014 - 10:36 AM