3. They can make donning/doffing the fins difficult.
The rubber straps aren’t particularly stretchy. When they’re adjusted right, you still have to pull on them a bit in order to get the strap over your heel and free your foot from the fin. When you combine this with the tendency that many divers have to over-tighten their straps, it’s easy to see why most manufacturers of dive fins have opted to include quick release buckles on either end of the fin strap.
These quick release buckles make things more complicated, though. I’ve seen several divers struggle to re-attach a strap buckle when they’re in full gear, as well as fumbling to release a buckle on the surface while holding on to a boat ladder while being tossed about by the waves.
Additionally, the clasps on the buckles can snag on things such as the line for your dive flag, bits of fishing line, or a navigation line.