I bought two "mostly complete" uppers when I got started.
All lowers, and every upper since the first two have been built from parts.
No kits.
As bedlamite put it: Frankenguns.
In one of them, a pistol, almost every single part is from a different manufacturer.
Edit:
Forgot to add:
The two purchased uppers
both had issues. (Company doesn't matter -- they folded two years ago.)
One upper that I built had a bad barrel (bad headspace) from Black Hole Weaponry (now calling themselves Columbia River Arms), and, after BHW kept letting me down, ended up with me buying a chamber reamer and finish-chambering the last replacement barrel by hand.
One upper had a bad extractor, but the same problem could have occurred if I had bought a complete rifle or upper from the same company. It was just an assembly mistake (wrong extractor), and the company went above-and-beyond to make things right.
Everything else has been great -- including the absolute 'frankengun' pistol. That hodgepodge has never had so much as a hiccup, even though pistols are supposedly notorious for being unreliable and difficult to get running.
I mostly use blemished receivers. I don't care about cosmetic defects, and I've only had a few where I could even
identify the defect(s) -- one of which was just white lettering for the T-marks on a receiver that wasn't supposed to get white lettering.
Otherwise, it's mostly reputable name brands for parts. -Not necessarily 'high end', just reputable and decent.