FYI Aspyr say the game is playable on the M370X, M380, M390, and the workstation-focused D300, D500 and D700. All of which, except perhaps for the D700, are outperformed by the 780M GTX from the 2013 iMac. These are mid-ranged mobile GPUs at best, and the 780M is a high-end mobile GPU, albeit from 2013.
Also, nVidia released the web drivers for Mac because they have workstation/professional users running Mac Pros who want to use modern GPUs (and optimise CUDA performance) in their professional applications (e.g. Smoke, Resolve), not play games. This is why it took so long to add support for non-Mac Pro systems.
The web drivers had crappy Metal support for a long time and the most recent Pascal-ready web drivers perform pretty badly, especially on Kepler and Maxwell cards.
It's safe to say that nVidia's Mac support is not as good as people seem to claim it is.
Yes, the AMD drivers were buggy (buggier) for a long time, but "better than AMD" doesn't make them good, especially since AMD is now (apparently) fixed.
Driver instability - albeit strange - is at least plausible, considering they're still happy to list it as playable on the el crapo M370X from the MBP.
I own Mafia 3 on PC. I'll download it (41GB *sadface*) and give it a shot on my 780M GTX. Not expecting much, but I'll report back.
I played for about 45 minutes before having unrecoverable graphics issues, it was impossible to save or even switch to another application, needed to Command + Q to quit. The game also looks absolutely horrendous at 1080p on Low. FPS was about what I expected, 30fps at said settings. This is demanding game.
That's a very unsafe assumption. Each game has its particularities.
It's about as unsafe as assuming that game not out = Metal broken.
Again, as I said before, it's also very possible that they simply didn't want to release three titles at the same time, so focused their efforts on F1 and Total War while putting the finishing touches on Deus Ex.