KOA is no different than individually owned private RV parks. Some are better than others. Some are very good. We have stayed in many of them and, so far, have not encountered a "bad" one. They are typically a few dollars higher than nearby RV parks of equal quality due to having to pay for the name.
And as far as the often quoted "keep on adding", I have never paid more than the price quoted on their web site. Some do have amenities they charge extra for, but we have never used those. All so far have had free wifi, although as with most RV parks of any name it is not always good.