Given that all the UPS's used in professional comms rooms and data centres are double conversion...and reliability issues are rare, I think you are perhaps a little over-sensitised by your friends experience!
I run a server 24/7 and keep it connected to a dual conversion UPS - it has been running about 4 years so far.
My older UPS has been set aside as it needs replacement batteries, but ran just fine (also dual conversion) for 7 or 8 years....
a dual conversion with a clean artificially generated sine wave output, guarantees a clean AC power supply for components connected to it.
You could be lucky, and be the first connection after the utilities Transformer, with no one else on that circuit - Yippee! - or you could be the last house on a long run, and get to enjoy the noise generated on the line by every other house on that circuit (ouch!).
Also, if you have well designed/built equipment, it won't suffer from noise on the lines....
On the other hand... I used to own a fridge in my student days.... (my grandmothers hand me down from the 50's!) - and every time the compressor turned on, it would reboot my PC (heaven help me if I hadn't saved what I was working on) - it also generated an audible thump on the stereo... Had the stereo been a digital component with a CPU, things might have been worse - but it was the good old days!
I've seen noise come through on power lines from neighbours fridges, washing machines and machine tools.
But nowadays, I cannot tell, as I have been running a Belkin UP60 (overblown overpriced powerboard, with surge supression and power filtering) for over 10 years... if something too drastic is detected on the line, it shuts everything down... which does happen every so often.
If you need a filter - then get a filter - but if you need a UPS, then you should get a UPS ie double conversion - otherwise you are actually running a filter and not a UPS - just with a few minutes of UPS attached on the end.