Well depends but as some say: necas own paintjob and quality on their stuff is a hit or miss thing and depends heavily on who in the factory put it together and painted it. Some are painted very badly while some are really well painted.
From 6 of my aliens warriors from neca, 2 have a terrible paintjob, 2 acceptable level and 2 which looks like they should. Only 2 have good joints and are perfect, rest have imperfections....
The knock offs: Most of them are made in the same factory after they closed, by workers making extra molds and then put it together either there if no one sees them or at their homes. And same here: if the person making it is good then the knock off is very close to the real thing, but then there are those who make a ton of those knock offs and is lazy and you end up with bad knock offs....
Then you have the true knock offs: people who make their own copies of these and they are even worse.
I also noticed:
Necas xenomorph figures and the more advanced figures often have worse quality than their more simple human figures where I havent encountered same amount of problems, my guess is they produce these figures in way too fast pace to allow those who work to take the proper time making them, and necas own quality control miss a lot, but thats a theory of mine.
