The problem is that "mistakes" is sometimes subjective.

Personally, I think their online system was poorly done and purchases being tied to consoles was a mistake, for sure. Both of those things are being fixed on the switch (supposedly).

Most people on /r/games think that not being as graphically powerful as the competition was the Wii U's biggest mistake and thus they're not learning their lesson with this. That's flawed thinking, in my opinion. The Wii was extremely underpowered and still did very well. The reason the Wii U did god awful in the first place was the marketing. That was a massive mistake, which, once again, they've fixed.

/r/games also likes to talk about how the Wii U had bad third party support and blame it's lack of power. As a dev myself I can tell you that had a small part in it (keeping it from getting games like the latest call of duty) but a much bigger part was simply the lack of sales and how hard Nintendo made it to port games to the Wii U.

Every dev I've heard from has said the Switch is very easy to port to (if you're not a graphically intensive game). I undertand that some may still think it's a mistake to not get those types of games, but by making the system portable they should more than make up for that market with all the portable gamers.