Indeed, that's one of the Wii's weaknesses. But is that weakness worthy of a double-LOL? While a Wii user may have to emulate newer MAME titles on a PC, he needs to do that for the full MAME experience anyway, regardless of which Wii\WiiU system he owns.
For 240p games that do run properly on the Wii, there's really no comparison. We're not just talking about a game here or there, but pretty much the entire 8/16 bit console generations + handhelds and some arcade. You see, the Wii has a secret weapon that goes by the name of "Sony Trinitron". The WiiU claims to have no idea who the hell that is, but trust me, it's lying and extremely jealous. It can only wish to look so good. So good in fact, I may sell my XRGB Mini and OSSC after I finish RBG modding my TVs.