I always thought Wii U failure was mostly due to bad games and lack of new games, mainly third party games. In my opinion, Wii U library is very mediocre that's why I find it a very weak system
However Wii U games sells very well on Switch, so lack of software doesn't seems the problem here. So maybe the problem is the hardware?
Unlike the uneventful software library, I don't think Wii U hardware is that bad. I think it's actually alright. Not something that stands out, at least not enough to make it such a failure. So exactly what about Wii U hardware made it so unappealing?