From my experience Colossus Slayer works consistently, if anything Colossus Slayer works TOO well, as in it also procs if you hit target at full HP. Clearly that is a bug and I guess it's because damage applied is in 2 separate portions as implied in the log, as in it probably first applies the arrow damage and then checks if CS can proc, but of course it can because target is already not max health.
Hunter's Mark does not work at all now..