It depends. As you probably know, even FreeBSD won't make an old PowerPC iMac a faster computer.
Also, hardware support (for various hardware that might be inside a PowerPC Mac) varies. The older the Mac, the more likely it is that everything is supported.
Other things to consider: not all ports will work on the PowerPC architecture.
Lastly, the install process can be an unknown adventure, as PowerPC Macs have their own firmware (not UEFI or Bios).
That being said, FreeBSD will be (as always) a stable and solid os if you get an installation you are happy with.