Interesting but you can't stop there :-), what are the pros and cons in your opinion?
NetBSD?
Pros (personal opinions, based on personal weird/wild needs):The kernel runs on every kind of hardware!!!
From 68k-mac (Quadra/LC475, 68040@25Mhz, 32M RAM) to PowerMac G2(PowerMac4400), G3, G4!!!
But also Sh4 (Sega DreamCast, Hitachi SH-4 32-bit RISC@200 MHz, with only Memory 16 MB RAM!!!)
It's well documented, and well written.
Cons (personal opinions, based on personal weird/wild needs):NetBSD, like OpenBSD and FreeBSD, doesn't come with a kernel builder facility, you have to manualy edit files to configure the kernel.
Being BSD != GNU, I haven't integrated Catalyst or Portage yet to reuse my GNU/Linux profiles for stage{1..4}, so the NetBSD rootfs is the one created by them, to which I simply add ( re/compiling manually) my things.
I haven't yet migrated/adapted special kernel drivers for CANbus and optical fiber (PCI 32bit 3.3V and 5V, to be used with PowerMacG4), which are "too Linux-ish", and never released for BSD.