I'm sure it's either a wrong version of the IOS the game uses (quick to test) or one of the shared content files is different from those people that have it working, or a third thing (unlikely) could be the install method.
It can't be the difference between a full nand dump and a partial dump, well unless you want to be really vague about it.
At present I've got a new Wii too and the art style games weren't working with my current setup. I had another Wii before this and they never worked no matter what sys menu Ver the emu nand had, app loader or not, save file on before etc. One day they started working though.
What I learned at the time, was no sys menu version change, full dump/part or save file "tricks" worked. PAL on PAL btw. I gave up because I didn't have time to continue going through every situation and I never got a comparitive from someone with it working.
Then, I went back to doing other things with the Wii and forgot about the games. That Wii was formated, virginised and set up over and over with testing some other things at that time and I cant be sure of everything that changed but I know IOS versions changed, the shared content was newer or older but NOT the same and the WiiWare were all installed to the NAND, played, saves made and THEN a nand dump made for Triiforce.
So that would be option 3, the install method..unlikely but maybe installing these games to the actual nand writes something different to the uid.sys, or it prefers one version of a shared content over another that ShowMiiWads doesn't.
The answer is there somewhere.