Would echo the sentiments that the instructor fraternity usually makes a great deal of effort to get people through. This is usually invisible to the stude and they may see the changes of instructor as "just being messed about and leading to lack of continuity", but often the staff are working hard to match studes to instructor to sortie profile. Trust me, it's not often that the stude knows what's best for them.
As to the carping, "well, I was good enough but there weren't enough slots" - no you weren't!!. You may have been good enough to have passed the course 6 months ago or a year ago but as requirements change, so the bar moves - how could it be any other way? On any course, if you are good enough you will pass - you might just need to be bloody good if your timing is off!