As the OP, I can confirm Toast 12 is working with Yosemite on my iMac. Heck, Toast 8 was working too, with a few glitches with the progress screen. The most typical cause of failure is the Mac going to sleep during encoding or burn. Before starting a project I head to System Preferences, then Energy Saver control panel and set it to 2 hours before sleep. I don't mess with the display setting, it dims in a couple of minutes. If I forget, I make coasters.
Yosemite 10.10.2, Toast 12.0.1, Mid 2007 iMac, 4 GB ram