I realize it's probably just paranoia on my part but I am terrified of UPSs and can't bring myself to have one in my house. Gigantic batteries lying around in a flammable environment seems way too scary. My always-on server is read-only about 99% of the time, so I just put up with the outages when they happen (about 2 a year). If for some reason my OS eventually gets hosed because of this, I'll rebuild it.
Maybe one day if have a basement and some kind of concrete compartment to put the battery in I'll feel a bit better about it... but even then, not much you can do about gas leakage if that's a possibility with more recent UPSs.
A single lead-acid battery is putting out such a tiny amount of hydrogen. Put it in any room. It's a big fat nothing in a fire too: https://www.youtube.com/watch?v=ndfe0c00gwo Something could short but shorts are possible and dangerous even if no batteries are involved.
Or get a LiFePo battery and put it in a metal tub.
It burns about as well as any random piece of electronics in a plastic shell. Any of those could spread too. If you want a battery that's less flammable everything else in the room I guess you're out of luck, but I don't think that's a reasonable standard.
Maybe one day if have a basement and some kind of concrete compartment to put the battery in I'll feel a bit better about it... but even then, not much you can do about gas leakage if that's a possibility with more recent UPSs.