This was similar to why PowerPC had a load of issues, with I believe two different official hardware architectures developed by AIM themselves, as well as legacy ones developed by IBM, and the proprietary Macintosh one developed by Apple. There was just no way to have your one system work the same on every machine like you can sort of do on an IBM PC compatible.
Ehhhh, not really.
The first generation of Power Macs (the ones with NuBus architecture) were essentially legacy 68K designs with PPC chips shoehorned in. They really are weird systems, with true "old school" Mac ROM, which literally cannot bootstrap anything other than classic Mac OS. (Any alternative OS for them, like A/UX or Linux, boots by first booting the Mac system software and then jump-starting it into the new OS via a warm reboot.)
There was a proposed standard PPC system architecture (PReP), but none of the AIM alliance members was actually happy enough with it to ever use it. So they each did their own thing initially.
But the later Power Mac systems (PCI based) are very close to the AIM's second standard PPC system architecture, CHRP. Indeed, they actually influenced the CHRP standard. These already made the move to Open Firmware (but still retain the Mac Toolbox in ROM).
The last major change to the Power Mac boot architecture was the switch to "New World ROM", which retains Open Firmware but no longer has the Mac Toolbox in ROM (for classic Mac OS, it's simply loaded from disk). These systems are
really close to CHRP. Indeed, Mac OS 8 actually had CHRP support, and the old steamer trunk of memories in my brain thinks I remember that, right before the Mac clone program was killed, the Mac was about to switch to using CHRP. (Some clone makers already had CHRP prototypes running.)
A Power Mac with Open Firmware (especially the New World ROM ones) is really
very similar to an x86 PC with EFI. They use the same hardware interfaces (PCI/PCIx/PCIe, AGP, PC
nnn series RAM, ATA/SATA, USB, etc.) and even support the same PCI cards, provided the card adds an Open Firmware ROM if you need bootability. (It's possible to have both an Open Firmware
and BIOS ROM on the same card, as a handful did.)
It would have been trivial to make one PPC operating system run on another PPC system. It was pretty much just fiddling with the bootloaders. PPC Linux does this.
Incidentally, what I find especially cool is how Apple managed to make dual-architecture OS builds, where a single Mac OS X install on a disk can boot both PowerPC and Intel Macs. (This was supported in 10.4.11 and 10.5.x, the only Mac OS X versions that existed for both architectures.) So not only could this enumerate hardware on boot, it could do it for two completely different architectures, and once bootstrapped, dynamically load the correct binary code chunks from the executable files. Except in actuality, it was
four different CPU architectures, since it had both 32- and 64-bit PPC and 32- and 64-bit Intel. This was incredibly handy for Mac technicians, since we could just install 10.5.x and our utilities onto a FireWire hard drive and use it to boot everything for servicing, be it a 32-bit PPC like a PowerBook G4, a 64-bit PPC like a Power Mac G5, a 32-bit Intel like the initial MacBook Pro, or a 64-bit Intel like the Mac Pro.