It's the age-old question. It's the complexity of the system you're emulating rather than the actual game type. We've been waiting a millenia for GTI Club because of some awkward protection.
GTI Club isn't protected.
You've been waiting millenia because none of the devs are good enough / have the interest to finish it.
Some guy did actually emulate it in a standalone years ago, but never released it (I had some shots of it on my old mame.net site back in the day, was full speed on my original Geforce 2 card too)
The MAME emulation is somewhat playable, but still very glitchy.
This thread is a mess anyway, I'm assuming the original poster is talking about which games are the most demanding on the host PC, not the hardest to emulate, despite the misleading title. The hardest to emulate stuff isn't emulated.