My experience is when I had multiple small UPS's , A) I consistently undersized them, giving me only a couple of minutes to respond with a power failure, and B) I regularly forgot to change the batteries on schedule, so one or more did not work when needed. When I moved my office, I put in a server room, and a BIG backup UPS with a pure sine wave inverter designed for solar systems, and a big battery bank to run all the computers in the office on a dedicated uninterruptable line. It cost about 1/3 of what a similar APC system costs, but took more Geeky interest, research and cooperation with the electrician. The end result, however, is that all computers in the office can run for over 2 hrs with a power failure, there is instant changeover (within 1/2 Hz), and only one battery bank to maintain. As I said it is definitely a Geek system, but saves a lot of anguish.
