They are not good.
I have seen a number of fakes in their slabs mostly of Chinese coins both modern Pandas and some of the earlier Republic stuff.
That being said, I have indeed even bought some coins in an NNC slab.
All the coins I bought in NNC slabs were indeed legit but I have only ever bought coins that I knew so well that I could authenticate basically from pictures only. This for me is meiji era Japanese coinage. I would never buy say a
Morgan dollar in an NNC slab because I don't know enough about it to authenticate just from seeing it in the slab.
Also, even if you know enough to authenticate by sight it should be noted that the slab CAN conceal some stuff such as rim damage or even in some cases things like mount removal - NNC in particular uses white foam type material to surround the coin so unlike PCGS for example you cannot see the edge of the rim at all.
Another thing to look out for with NNC is that they tend to grade cleaned or whizzed/polished coins because especially for a beginner figuring out that a coin is cleaned can be quite hard and polished coins often look really nice without very sharp eyes or some kind of magnifier (and these guys have been known to deginate polished coins as prooflike (PL))
So if you really know your stuff you can get some absolute bargains on
ebay with NNC slabs because many people know NNC is awful and will bid the coin as raw or even assume it is fake if it is rare.
You won't be seeing many non
US coins in NNC slabs fairly soon as about a decade ago they stopped authenticating world coins basically because they weren't good enough at authenticating them. Since then most of the legit world coins were reauthenticated at NGC or PCGS because no collector wants to keep them in that slab - for one thing the foam stuff in the slab is probably NOT inert.
So even if you are an expert authenticator and bought a bunch of bargains, you should almost certainly have them reslabbed just for the sake of preservation if nothing else.