I think where they go wrong is by trying to trick people into extra fees. Like making the interface counterintuitive to book hold bags for both legs of a return flight, in the hope that you'll forget to book on return and pay a fee. Or charging a fee for checking in at the airport (even if it's using the same self-service terminal you need to use to print bag tags for hold baggage).

They definitely aren't the only low-cost carrier to do it, but they seem to be best at it. In talking with people IRL they are definitely the carrier that comes up the most when sharing anecdotes about shady fees.

In comparison, I flew with Volotea a year ago, and our hold bag on return was about 500g overweight, so the self-service bag drop refused it. An attendant came over immediately and checked it in manually at no extra charge. I wish there was an airline that combined Ryanair's excellent network and efficiency with that kind of customer service.

Also, while it isn't something their passengers necessarily care about, Ryanair is quite brutish about unionization of cabin crew and ground staff. I think they treat their pilots OK since they don't really have a choice, but they squeeze their employees everywhere else they can.