Hah, I really debated a refurbished UPS for like a month because I was afraid of battery capacity
Bit the bullet and got one like 5 years ago.
Still going strong. No idea what the capacity actually is, but it can power my router and modem for about 4-5 hours. Which is what it could do 4-5 years ago when I bought it.
I didn't mention it as an example because honestly, a UPS sounds like something you shouldn't skimp on and I figured I was just lucky.
But it makes sense, on a manufacturer refurbish they replace the failed part, then test all the other main components and the system as a whole. So less likely to have any other flaws.