Finally!
I upgraded from Mavericks (not fresh install) and am 100% functional (except for USB 3.0, will tackle that another day).
I had a Mavericks install that was flawless. Yosemite completely destroyed it. The old tricks simply didn't work. Good news is I don't need as many hacked kexts with Yosemite. In the end the installation is actually much simpler.
System:
GA-Z68X-UD3H-B3 Ver 1.0
OS on Crucial M550 SSD
Radeon HD 6850 Video card
First problem:
I could not get Yosemite to run with old-school bios (F10-F12). At that time I had multiple issues. No sound, no network, boot hangs, you name it.
Installed UEFI
Support & Downloads by making a boot USB running FreeDOS
http://www.freedos.org/download/
Go into the new bios an enable ACHI etc.
Booting into Unibeast with the USB was never an issue. Just make sure you aren't trying to use one of the USB 3.0 ports. At some point i did have it so crossed up I had to boot in safe mode (-x) and use the disk util to do a permission repair
right before installing Yosemite. Otherwise the install would fail because it couldn't unmount.
Once installed, I could always boot on the SSD in safe mode.
Most issues were caused by using too many of the old options and kexts in Multibeast.
In the end I used:
The only modified kexts on the system right now are:
AHCI_3rdParty_SATA.kext
AppleACPIPlatform.kext
AppleHDA.kext
AppleHDAHardwareConfigDriver.kext
AppleIntelCPUPowerManagement.kext
FakeSMC.kext
IOAHCIBlockStorage.kext
JMicronATA.kext
RealtekRTL8111.kext
VoodooTSCSync.kext
Most problems with the upgrade install seemed to come from hacked kexts hanging around from the Mavericks install. Once i got down to just the ones above it was good to go.
It initially would not boot without -x or -f boot flags. Fixed this with a rebuild of the kernel cache:
sudo kextcache -prelinked-kernel /System/Library/Caches/com.apple.kext.caches/Startup/kernelcache -K /System/Library/Kernels/kernel /System/Library/Extensions/
The RealtekRTL8xxx.kext that used to work for Mavericks no longer worked. RTL8111 works with Yosemite.
I also no longer need the hacked bluetooth kexts to get the BCM20702A0 bluetooth adapter to work. If you leave them in there bluetooth will
NOT work.
The thing boots like lightening thanks to the UEFI.
Survived another major OS revision!