I've got a MBA Mid 2013 and since installing Yosemite, I've had 3-4 sleep wake failures. Generally I'll close the lid, head home and either when I open the laptop at home or back at work the next morning I get message (not sure of exact text) "Your computer failed to wake properly and has restarted". It didn't happen with Mavericks nor ML before that.
2 Times it stated the process was Activity Monitor and this most recent time it was airportd.
I don't really lose anything since all my apps open pretty much where they left off it's just a big annoying.