Luck has a lot to do with it as well. One of the bigger FE builders, that is mentioned on Club Cobra all the time, and who is well-respected, gets absolutely blasted on my local Cobra forum because we've had a couple of members that, for whatever reason, have had bad experiences with that builder (who eventually made everything good, it just took time). And that particular builder has successfully built probably a zillion FE motors. Think about it, if you've done anything a thousand times, you've probably done it poorly at least a few times out of that number.
But around here, we expect engine builders to be like 767 pilots on a landing -- you only get one chance to do it right, and you damn well better do it right, because you probably won't get a second chance. Oh, and you have to do it right every single time, no exceptions.