My personal experience: Three times a destroyed virtual disk (root partition of a small more or less static webserver with not much load). The boot then ended in the GRUB rescue shell because the filesystem had gotten some serious damage. I don't know what others do differently, I just followed the instructions of PVE/Linbit.What is so risky about it that many other are suggesting not to use it yet? And when should be safe to use it? Could it take months?
I talked to one guy at Linbit, who was a little bit surprised that I planned to go for a productive system with DRBD9 in its then actual state and he wished me good luck, which unfortunately didn't happen ;-)
I read somewhere that they plan to reach final state by the end of June - but I don't find this info anymore.