I haven't had one fail in such a way yet myself.
With an online UPS, the load is always running from the batteries, right? So if the batteries give out (e.g due to being too old - you should have replaced them months ago), can that cause loss of power, where the mains itself is fine ? Or does old batteries simply mean the autonomy time is poor ? Or maybe the online UPS can quickly switch the load onto raw mains if the batteries fail. Or something.
I worry that you protect against one type of problem, only to become vulnerable to another.