Because Apple just released the Gold Master for Mountain Lion two days ago - nearly a month after the release build of AC 16, it is possible that 16 (and 15) will require hotfixes if there are compatibility issues. It is possible that Mountain Lion will be available for download in the App Store before Graphisoft has their hotfix (if any) ready - because of the time it takes to go through the quality assurance process. Guess we'll see.
As many of us recommended last year with Lion - make a complete, bootable clone of your old OS before installing Mountain Lion just in case you must 'roll back' for some reason so you can avoid any major loss of time / productivity - whether related to ArchiCAD or any other software you run. External hard drives are cheap, or you can partition (one of) your internal drive to include a cloned bootable prior-OS. (I've kept a bootable Snow Leopard partition around in case I need any old PowerPC [PPC] apps - including ArchiCAD 9 or earlier, none of which can run under Lion or newer. Surprisingly, I haven't needed to boot into it once, but it's a nice safety net having the option.)
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sequoia 15.2, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB