I have experienced sleep crashes both plugged in and unplugged, tho most often when plugged in for me. It is not the hand off to hibernation that causes it, as my machine is set to never hibernate when plugged in, so there's no reason it should be even trying to enter that state when I experience the most sleep crashes.
I wouldn't be surprised if the cause is legacy applications, but I do have to say, I leave NOTHING running on my SB when I put it to sleep other than what always runs in the background. I do not use or have iTunes on my machine.
My sleep crashes also seem somewhat random -- that is, I don't get them with any consistency, and I've yet to notice a distinct pattern (particular applications that were used prior to the sleep session that caused the crash, particular errors in the logs prior to the crash, etc). It can be days without a crash and then one day, I'll have it happen every time it sleeps. It's just so frustratingly random.
I built a small form factor desktop a few months ago after I realized I just couldn't rely on my SB for all my computing needs (too unstable) -- with a skylake chip. Amusingly: no sleep, no hibernate... and no issues.
Back to all-hibernate for my SB for now ...