Other three are comman senses, for safety and health concerns,

for drawbacks if the bedroom is on the top of garage, I think it is more becasue structure wise concerns than fengsui wise.

I am buying people saying, if your bedroom is on top of the garage, usually means you are on top of a cold box in winter or hot box in summer, because usually garage is not insulated well enough to keep temp as other living area. It may drag down or up a lot on your bedroom temp, make it not that comfortable to live in as compare to other locations.

For FengSui my standpoint is if I can't know them all, I am better off not to know much than I should, otherwise, life will become really mysterious.