"... I've not had any T1 failures of the magnitude of it "shutting down" in mid song..."
I performed with my T1 this past Sunday, outside, in fairly hot, humid conditions, for three hours straight...no breaks. It seemed to work just fine.
However, I do have a problem with Ch4/5 as it turns out - and had I been using Ch4/5 I would have experienced a channel shutdown for sure! And it likely would have taken place about three to five minutes into the first song!
For some reason Ch4/5 cuts out after a few minutes, for no apparent reason. I found that if I snap one of the bass strings on my guitar (pull up hard, away from the neck, and release) then Ch4/5 starts working correctly again. Almost as if the noise gate was turned on, and by snapping the string I was playing with a sudden increased volume, overcoming the noise gate threshold setting - except that the noise gate wasn't on. It was bypassed. I say almost as if, because if it was a noise gate issue I'd have to snap the string to get the channel to 'open up' every time I tried to play something through that channel. So is it noise gate-related? I don't know.
Fortunately I didn't need Ch4/5 on Sunday so it wasn't an issue but I will be checking in with Bose Customer Service to see what the problem could be.
As for the built-in Tuner: mine wanders all over the place as well. Although I've yet to see an electronic tuner that doesn't. I have a very popular Boss chromatic stomp box tuner, and one of those Intellitouch tuners that clips to the headstock of the guitar, and both those wander all over the place as well.
I found that if I muted the five strings I wasn't tuning, with my right hand, and plucked the string I was tuning very lightly, I could keep the T1 tuner's 'wandering' to a minimum.
Anyway, I was able to use the tuner feature in the T1 for the whole three-hour performance and I was able to make it work well enough to not require the use of the other two tuners I have. It certainly was no worse than the Boss or the Intellitouch tuners. Though it would be nice if it was a bit more stable. Fortunately I didn't buy the T1 for the tuner feature. However, in a piece of equipment of this quality I would expect the tuner to be a bit more stable with respect to 'wandering' etc.