We've been during every imaginable crowd level. Never noticed a single different in bus service. Disney's buses run pretty much continually, showing up every 20 min or so. They don't show up less when there are fewer guests. There are however, glitches and sometimes they don't quite make that cycle, sometimes they are faster.
Sometimes you have good timing and you show up just as a bus is arriving.
Sometimes you have bad timing and you show up just as a bus is departing.
That can be 5 min difference yet make a HUGE impact on your perception of the bus service. If you are in mostly in group A, you think the resort in question has the best service ever. If you are in group B more, you think it's the worse. And each group can be there at the same exact time only hitting the same exact bus stop a few minutes apart.