I've built , and participated in the past in builds with Imbel, Enterprise, Coonan, DSA , Unibrow Century receivers. this is what I've either experienced or witnessed personally.
Timing issues:
3 DSA receivers so bad they had to be shaved as they timed at around 9:00 that's 3 out of 3 DSA builds! 100% fail on builds I did .( they had to be re-machined and threads chased to finally work)
1 Imbel non gear logo that timed at 11:00 that but was extremely tight, I finally timed it TDC with a breaker bar on the receiver wrench after much effort. ( built 3 Inbel, and witnessed / participated in builds of another 10 or so.)
Non issues:
Enterprise- 4 rifles built with a friend on them with no problems.
Coonan - built my first G1 on a type 1 Coonan with no issues at all and one of the most accurate FAL rifles I've ever shot.
Minor issues:
Unibrow Century- participated in a build of 4 of these and witnessed several other home built ones . Most ran fine, issues included minor timing, operating rod hole issues that were fixed by opening p the hole slightly, and 1 failure to go into battery due to excess material that was easily fixed with removal of a high spot .
IF I ever could build one again I'd use Coonan, Imbel or Enterprise with confidence.