Mac OS X binaries are the most susceptible to bit rot of any major OS. There's constant platform churn, all of it closed source, and there's nowhere near the community that exists around emulating older Windows.

You can reliably run DOS, Win, Linux applications of almost any vintage with the right translation layer / emulator, but there's a black hole of MacOS apps circa 2002-2012 that just no longer runs on anything, even later Intel Macs.

I used to enjoy a cute little Pacman clone called Pac the Man X, for example, and I just don't know how I could run this today. Seems to reliably take down any emulator I've thrown at it - presumably it's using some kind of graphics acceleration that's not emulated. (Suggestions welcome.)