They are very rare, if not non existent in my area. I RO and I've been a dedicated RO/CRO at regional matches. I don't think at a Section or State Match I've ever seen one.
At a local match, I've seen a few. Maybe 4-5 times in the last few years.
Some people come out to a match just to see what it's all about. Some of those people might continue, some are one and done. I can't recall seeing anyone that is a semi-regular or regular competitor that uses one. For the game, a retention holsters is not ideal.
As a safety issue. Sure, I think anyone can point out the safety concerns. For USPSA 1, they have a rule that says they won't dictate what type of holster to use but they can disallow a holster that is unsafe. Usually that has been applied to things that aren't even holster or some version of cloth and string. Again, for the first time person that just showed up. As for Serpa....I just don't think it's on their radar. They are so uncommon and maybe because nobody has shot themselves in a match.