For small loads it's perfectly fine. Big loads would push the duty cycle of the transformer and the excess heat would be an issue but for small loads like a router or NUC it's perfectly adequate.
Anything bigger like a desktop PC or server in active use I'd upgrade to a bigger UPS to ensure a better duty cycle on the components, but that's also an additional expense.
As others have said, buy a bigger ups. You can then take your old one and move some of you other gear to it, which make the new one run your server for longer. Ideally something with auto shutdown software.
This is what I ended up doing. I am on my third ups now, and my oldest one is now protecting my desktop.
Don't use car battery in UPS. They are made to start car, high surge current. Under deep cycle in UPS, battery will be ruined in a dozen cycles. Get a 12v 50-100ah LFP (LiFePO4) battery. Wire it to UPS. Should do the job. I retrofitted multiple UPSes with quality LFP. Much better
If the model allows for expansion or chaining battery packs, of course. Do not use a car battery unless you want to die. Best bet is buying a bigger one.