I have broken a few over the years, but it happens in two situations:
1. The buckle is snugged up tight on a tank on the second dive where the strap is fully soaked and more prone to stretching, then the strap dries while still attached to the tank. The resulting contraction of the nylon strap can be enough to break a plastic buckle if it was tight to begin with.
2. The buckle is extremely cold - for example while gearing up for taking the gear apart after an ice dive with surface air temps pushing zero. When cold, and under tension, it does not take much impact to break a plastic buckle.
For anything technical diving related I use metal cam buckles. I like the Scubapro design, especially with some minor modding to keep the end of the tab tucked in so it does not create a line trap. The Dive Rite design is a bit high profile, but their newer compact buckle looks intersting.
For warm water recreational diving, I have no issues with using a plastic cam buckle, but I also use two of them even on a single tank, so there is 100% redundancy and because there are two of them they do not need to be quite as tight. That is not usually an option with, for example, the cam band used on a sidemount tank, so in that case, I'd revert to a metal buckle even on a recreational dive.