i played through the Reach campaign already and didn't notice any issues with the cut scenes. they're locked down to 60fps even if you have the fps limiter unlocked, but they still looked fine to me
i'm not too sure why, but i've heard a mixed bag of things. the most common answer is that they're still using the older game engine (not sure if that's right or not) and it takes a significant amount of time and effort to rebuild it and make it work for modern systems.
to be clear, the game still plays very well and i don't have any complaints except for the fps smoothing at higher frame rates. it's also pretty sweet to play CSGO with the halo music track haha. i grew up with Halo and the soundtrack for the original game will always be at the top of the list for favorite game music.
unfortunately i didn't get a chance to play Halo on PC. after playing against console players on the new Modern Warfare, and also playing some halo on Xbox recently, i think it's important to recognize that console players will always have the auto-aim feature for their controllers. it doesn't replace the mouse and keyboard mechanic, but they at least have something to help make it even for cross-platform play. there is a huge difference between the two, but console players who've been playing console fps games for a long ass time will still definitely be able to compete with average/casual PC players