I feel that the 58-pattern guns can be taken fairly seriously and I would not hesitate to use one as a fighting gun. In fact I took one through Ken Hackathorn's advanced carbine class and it performed extremely well. There were several there; no hitches at all. Mine would feed empties as well as loaded cartridges. Doing malfunction drills was actually difficult; it was hard to make it stop running. They are not prone to the issues of the AK or the AR at all. However I have zero knowledge of the .223 versions and could not comment on whether they're worthwhile at all.
I consider Kel-tec stuff to be entertaining at times. I kind of like their ridiculous shotgun. I would never, ever trust their stuff for serious use.
I would put a good x39 VZ or 858 against any AK or AR for reliability. With a few tweaks they run very fast. The only real disadvantages, IMO, are the somewhat fragile, rock-and-lock mags, non-chrome-lined barrels on the NR ones for the most part, and varying spec on the different versions (which I suspect leads to the so-called gremlin). And I'm not a huge fan of x39, although I do have crates of the stuff on hand (hey, it's cheap).