What UPS brand plays best with Linux?

They can connect via USB so you can do things like perform a clean shutdown when it loses power

Yep even mid range ones did so way back in the early 90s. It was a common fairly cheap feature that used rs232 mostly. In those days non unix systems did not care as much if powered down without warning. So us early (me 1996) Linux users really found it useful to have the ups initiate shutdown before it died. Windows users of the time only tended to worry if they had the PC running when not sat their. Few had internet at home in the early 90s and those that did were dil up. So for many it was not worth the cost when you could just turn off the PC if a power cut was long term.

Now with most people using laptops for casual use. I really cant imagin anyone buying one without the feature.

If the computer gets notified that the battery has been activated, then it can save files, gracefully exit programs, etc. before power loss

It's possible for them to tell the computer to shutdown when the battery is getting low

I have 3 cyber power UPSs on my network and I haven't had any issues with them. The web interface isn't the best but it does it job

I was happy with my cyber powers for years, but then the batteries died (official replacement batteries, after 3 years - the originals lasted 5) and the ups just stopped even passing power through. This is someone's old blog about this https://blog.networkprofile.org/cyberpower-ups-avoid/

...and what better brand than EATON, which sponsors the NUT project IIRC.

Agreed; I have two Cyberpowers that NUTS works well with. That said, I hate configuring NUTS. It's like the sendmail of power supply management systems. I'm not aware of any others, though, so it's what there is.

Oh, and Home Assistant plays really well with NUTS!