We have an XGS87 which has experienced lock-up and reboot issues since SFOS v21 came out. Sophos is working hard on finding the problem, and I give the trouble-shooting personnel full credit.
BUT it has led me to believe that it's a crucial mistake to buy an appliance without a dedicated storage drive. Why? You don't get on-device reporting, but you can use Sophos Central for that. So...
THE issue is that debugging a crashing issue is going to involve Sophos personnel logging things and the XGS87 (and I guess second-generation bottom-of-the-line appliances) doesn't have a separate storage device. So: a) some logs are lost, b) after a reboot you can recover some logs but you have to act quickly, and MOST IMPORTANTLY c) if the logging is extensive enough to fill the tiny pseudo-disk, it will halt everything and rebooting will not fix this and you eventually have to go in via the serial console and find that log file and delete it. (You can boot off of the other image, but you need to rescue the one in which the disk went to 100% usage.)
So I would never buy an XGS without a separate, and reasonably-sized storage drive. It makes marketing sense, I guess, for a small branch office, but if anything serious goes wrong, you're hosed. For example, the current process logging that Sophos is doing to figure out the kernel panic will fill the pseudo-disk in 24-30 hours, which will in itself take down the appliance.
(I also have a suspicion that something in SFOS v21 is not happy with diskless appliances and so the XGS87 is incapable of running SFOS v21, but that's just a suspicion on my part at this point.)