I have this issue:
I use iMac12,2 because i have sandy bridge processor/motherboard.
With Yosemite and UEFI Safari crash every time i play an youtube video or any video is played with HTML5
If i change to iMac13,2 Safari don't crash but the video is freezed at 0:00 seconds.
I have found this problem is related to my smbios configuration.
My temporary solution is to set the UserAgent on Safari to 7.1 and the video will play with Adobe Flash and there is no problem...
but i need html5!
All works perfectly, iMessage and FaceTime works, ecc.
I have only this problem...
Anyone can help me?