I have owned three CETMEs. I still have two. None of them have failed to fire commercial ammo, none of them have gone KaBOOM, none of them have done ANYTHING other than place lead in close proximity to where I was pointing the rifle.
CAI in fact did cobble together a LOT of CETMEs from parts kits. Most of them are just fine. Some of them are not. Earlier builds had wood furniture and later ones had the synthetic furniture. There are, as best I can tell, at least three generations of CETME builds by CAI.
The issue with the CAI builds is that CAI tried to press some worn-out receivers (front trunnions, actually) into service when they should not have. These worn trunnions allowed the rollers in the bolt to stick out too far, and the telltale bolt gap to disappear. CAI compensated for these worn out trunnions by milling the back of the bolt flat to bring the bolt gap back into spec. But the gap is a metric of trunnion health, and the trunnions should not have been used.
It is very easy to tell if the CETME you're looking at is safe/good/worth its cost or not. Remove the magazine and flip the rifle over so the magwell is facing up. You will be staring at the bottom of the bolt and bolt carrier. If the back of the bolt (where it bottoms into the carrier) is completely flat, do not buy it. If the back of the bolt has a chamfer at the bottom, it is likely fine and you then should check the bolt gap. If you can slam home the bolt and then barely slip a paper business card in the gap between the bolt carrier and bolt, you are absolutely GTG. If you cannot, then you are OK but you'll may need to replace the rollers to bring the bolt gap into spec. This is a very inexpensive fix.
That's it.
On the plus side - they shoot 308 (just stay away from the Hornaday Light Magnum and you'll be fine; I like the Prvi stuff for moderate power loads), mags are dirt cheap, and the rifles are surprising accurate for military hardware.
On the con side - the ergonomics of the HK91/CETME rifles are an acquired taste (most think that the FAL and such have better ergos), and they're not light. There aren't many cheap ways to get a side folding stock, if you're so inclined, and scope mounting can get creative.
There is no reason to be afraid of CETMEs and many reasons to think well of them. You just have to respect them for what they are.
Oh, and you will find the occasional mag issue related to the fact that CAI had to (by ATF edict) replace the G3/HK91 magazine paddle assy with a button mag release. The CAI drunken monkeys didn't always take the time to properly chamfer the bottom edge of the new mag catch, and sometimes you have to remove this bit and do it yourself to get the mags to insert and release cleanly. Doing this detail allowed me to use allow G3 mags in my CETMEs with no undue force needed to seat the mags, allowed the mags to readily drop free, and stopped the gouging that the CAI mag stop was making in the light alloy G3 mag bodies.