But then your ATS becomes another potential point of failure. If you have so little trust in your UPS that you need an ATS behind it, I think you are probably better off without the UPS's.
In my last job I had around 100 APS SmartUPS's (mostly 3KVA) in various wiring closets and in the ~3 years that I was there, none of them failed unexpectedly, but we had to replace a few batteries and/or entire units when they failed their monthly self check.
Sure, but I've never had one of those ATSes fail. For a sample size of ~15 over a decade. Where I've pretty much had every APC BackUPS and SmartUPS (mostly 1500RM 2U, some 3000 3(?)U, some 2200 floor units) drop power at some point. That includes being very liberal with battery replacement, the last one was <3 months with a new battery. I would say that they seem to be reliable within 3 years, It's outside of that 3 years that I've had problems.
In my last job I had around 100 APS SmartUPS's (mostly 3KVA) in various wiring closets and in the ~3 years that I was there, none of them failed unexpectedly, but we had to replace a few batteries and/or entire units when they failed their monthly self check.