I have one of these APC Back-UPS 900 (BR900GI) here, and they work perfectly. Fully supported under apcupsd, that device has saved my bacon quite a few times. The only thing is that you MUST use the USB cable they provide.
This isn't to say it's good, just that it's not new.
It works perfectly with apcupsd though, even showing up in my desktop power settings as a UPS.
That was before USB was available so they used regular COM port communication, I'm not surprised with that early foothold there was no reason to change and sufficient force to continue ever since.
At the time it seemed like the un-necessarily non-standard RS-232 cable was just to create a "profit center" from the cables themselves, along with the non-standard replacement batteries. They sure would have been a lot quicker to market and had lower up-front engineering costs if they had used standard batteries from the beginning.
It's good to recognize early when excessively high TCO is the primary feature around which a product (or company) is designed. There can be significant PR effort from the beginning to divert any perception of anti-consumer attitude.
Lots of stealth can be involved to muddy the comparison with alternatives which offer normal TCO or well-engineered low-target TCO.
The serial port serves dual-purpose. Along-side the obvious serial, you can also provide some inputs by grounding certain pins, and some outputs by sinking current from others. This has got me through some unorthodox integrations by having the UPS signal that it's on battery by having these "simple signals" close a contact/relay on the load. It's not quite a full suite of dry contacts, but it's pretty serviceable.
The problem is that 9 pins doesn't leave many spare, so to achieve this they've repurposed some. I really wish they'd implemented this as an alternate mode so you'd, for example short tx to rx to change the behaviour - instead of having the 'alternate behaviour' sitting on standard DTR/RTS pins.
Getting industrial interfaces at SOHO prices was awesome. The unintended consequences .. less so.
(Generally, such a protocol is a better option from a compatibility point of view: it's usually fairly easy to reverse engineer. It would be interesting to sniff what the windows software is doing to work out what protocol or behaviour difference is responsible)
(I'm not convinced that anyone has ever been behind the wheel of the ship that is called APC.)
OTOH, the older UPS behaves much better with apcupsd, going so far as giving me the firmware versions, battery replacement times, better power monitoring/etc. While the new one works its missing most of what the older UPS could do (although its not a APC).
So, i'm sorta on the fence about just using "backup generators" from Amazon/etc because one of the ones I have has about 5x the capacity of the new UPS and seems to be able to switch without the computer plugging into it having an issue. Plus, its power waste is less than 1%.
The batteries are definitely worth more than the unit is at this point.
A backup generator for me is not an option but this is mostly just to keep internet and a couple machines going in the event of a power failure. One day maybe the used lithium ion one will pop up and I will swap it out.
I see people dumping old jackery, ecoflow, models for 1/3 of retail on craigslist/etc somewhat regularly although those tend to be larger models. Its a bit of a mystery to me why some of them aren't advertised as having UPS functions if they have fast switch times, which many of them do since they don't have physical relays. My old line interactive UPSs are all physical relays which click when they switch over, the assumption is that the energy in the transformer is sufficient to cover the switching time, but of course they do brown out a bit when near their current limits.