Well, the issues I'm aware of: Ryanair pay their staff poorly. And that is not cheap enough, so they also employ them under employment law from "cheap countries", to try and circumvent income taxes and other charges of the local countries they service.

Ryanair demand subsidies from local governments to fly to far-flung airports. They can hold these governments hostage, as often the airports fail if they stop their service. Of course, this could be seen as simply "bad management" by our local governments.

And to finish, though this is something that applies to all airlines : no VAT or "eco" tax, that the rest of us have to pay.