For me personally, WiiU is a great console:
- Unlike many people I thoroughly enjoy Nintendoland, and to this day it's a game we regularly play at gatherings, especially Mario Chase and that ghost thing. Nothing on the market allows such fun asymmetric gameplay, except for maybe Keep Talking And Nobody Explodes and Overcooked, and thats the number one reason why my WiiU is still connected to the TV.
- Bad as it is, virtual console on the WiiU still has some interesting titles, and as someone who regularly plays SMB1 & 2j this is great.
- NES remix, MK8, Zelda. Latter two are also available on Switch, but that doesn't lesser their value on WiiU, especially since I've been playing MK8 for years.
- WiiU gamepad is the least painful "portable console" for my hands. DS, 3DS, PSP and Vita give me the cramps after more than an hour of gameplay, while I could play on U gamepad for hours straight.
The biggest disappointment for me was weak gamepad signal which prevents me from playing in bedroom in my new apartment (it worked ok at my last, small, place), and the inability to play Wii games with just a U gamepad.
The limited success in my opinion was due to lackluster support from both 3rd parties but also Nintendo. While it has a bunch of great games now, waiting for them to materialize was a painful process. Also, usage of U gamepad was not standardized - some games can completely be played on the gamepad, while others required a TV. Not ideal.