does anyone use UPS for their low power equipment, such as dac and streamer, or CD/DVD player?
I do, and it’s a double-conversion 1500 VA UPS (always on battery power, with zero transfer time).
SmartOnline 120V 1.5kVA 1.35kW Double-Conversion UPS, Tower
Even though this UPS incorporates surge protection in its design, I have placed a Tripp Lite Isobar Ultra before it, along with a Furman PM-8 Series II Voltage/Current monitor (with Series Mode Protection - SMP) after that (to protect the electronics in the UPS). Those two devices in series then feed the UPS. I’m all about protecting my gear as much as possible. Post UPS; two balanced AC transformers then take that UPS power, and convert it to balanced AC.
I would expect the inverters in cheap battery backups to be noisy
If you’re referring to AC THD; when I owned a Fluke 43b Power Quality Analyzer, with nothing plugged into the UPS, the THD was 0.0%, with a perfect 60 Hz waveform. It just doesn’t get any better than that (as far as I know). As gear was plugged into the UPS, the THD did rise, but never exceeded 2%. If you were referring to physical audible UPS noise, well then yes, there is some of that, so in my case, it is located out of my listening area.