The GS collision system works "flawlessly". The fact that USE_BOX (and thus c_move) uses ellipsoids instead of boxes takes a little to get used to (and I often wish we had AABB again), but it does not make anything impossible or exaggeratedly hard to achieve.
Some time ago I've had problems with my movement code and I blamed the OBB sytem, so I wrote a full (2D) AABB movement function. It has worked fine, but it did not really solve any of my issues, so I thought again and finally (it's approx. my 8th gravity code for the same game) came up with a very easy (OBB) solution.

In general, this is not directed at you, Redeemer, I don't get it why people think a good movement code can be achieved with a few lines of code in a few hours/ days.