This discussion has been had many times, the issue largely has to do with insufficient head space for the high krausen with highly hopped beers, all that "stuff" ends up in the lid trying to get out, and PRV's/posts offer little exit, if they are side by side you are pretty much guaranteed that if one blocks then so will the other, no luck involved just the way it is. as previously stated there's a reason for blow off tubes, and it's only really the advent of cheap, suspect quality PET pressure fermenters coupled with brewers unfamiliar with pressure fermentation that this whole thing has become an issue.
End of day just look at at a couple of pressure vessels, your Bunnings compressor, the kitchen pressure cooker, your hot water system, all these things have a fail safe, but only the compressor and hot water system have the equivalent of a PRV, why? because they are dealing with a clean source, air in one case and water in the other, introduce solids (pressure cooker full of vegetable soup) and a different approach is required, in a ferment with solids be a it hoppy beer or a fruit laden mead a PRV or post supplied spunding valve will have the potential to become blocked and compromise the integrity if the pressure vessel.
As to why anyone would want to pressure ferment from day one is a whole different discussion