You can use SSDs on the array, I have a small one I've been using for may months, like mentioned trim won't work so write performance might suffer some with time, also recommend having a faster device (like an NVMe device for example) for parity (also with higher endurance) since parity will have much more writes than the array devices, the more array devices you have the more parity will suffer in comparison, and parity can never be trimmed even if/when Unraid supports array device trim in the future.