Is what safe to use?
If you simply mean beta 8 then it is certainly as safe to use if you are using base unRAID features.
Assuming by base features you do not mean disk replacement expansion. It does not support replacing a smaller disk with a larger disk.
Well sort of. You can replace a smaller disk with a larger disk, and your data will still be safe, right? You just won't get any of the extra space automatically at this time. Annoying for sure, but I'd still consider it "safe to use" in the context of that particular flaw, although it is obvious that there are different opinions regarding what "safe to use" means...
Speaking of beta software, it is certainly the case that things that “used to work” in the stable release might stop working in a beta.
After running betas flawlessly for a while on a dedicated lab computer I decided to try 6b8 on a more normal server that previously ran 5.0.5 flawlessly. Didn’t start too well, of course… Long story short, two disks were missing after booting 6b8, and I had to chase the problem for a bit before I could have everything up and running. Was tempted for a while to revert to my backup…
So what went wrong? The server in question has a Gigabyte Z87X-UD3H motherboard, and the two missing disks were both connected to the Marvell 88SE9172 chip. Combing the dmesg output, the kernel indeed detected the disks when booting, but then lost them after DMAR errors. Upgrading the BIOS didn’t help, but disabling VT-d in BIOS made everything work nicely.