More software probs than hardware. I see a fair number of 870s, since I,A, get people started and many of them buy an 870 after reading what I and others say about them and B, oodles of folks have them and shoot them where I do.
I've not seen a real dog yet. I HAVE seen.....
Off center choke threads.
Off center choke and barrel borings.
Less than acceptable wood to metal fit.
Some rough chambers.
The first two can be made right under warranty. The third can be lived with and the last can be fixed with 4/0 steel wool and elbow grease.
Old, used 870s are not as cheap as when I started buying them feverishly, but they still are a great value. Everyone should have some.
As for plastic parts, there's no difference in reliability, durability or ergonomics.
I PREFER plastic followers. They do not peen the receiver as the steel ones can under heavy use with buck and slugs. My oldest shows a trace of this after a mere 15K or so. My TB has close to 20K trap loads through it in my hands alone and shows not a trace of peening.
I've yet to see ONE of the plastic TGs glitch, and I'm sure I'd hear about it. BTW, for the weight conscious, the poly TG is about 2 oz lighter than the one made from recycled beer cans.
And while I like lots of shotguns, if I hit the Lottery the first thing I'd buy would be more 870s, including letter grade trap guns and a 28 gauge WM.