The few times I have needed it to come on... it has.
Like the first time I came out of a roundabout in the wet, without thinking about it, 2-3 days after I had it, driving it like it was FWD...
I got just a slap on the wrist to say... "Calm down dear" ... Which I am VERY grateful for, as it has all ways done what I needed/wanted it to.
But nine times out of ten, it comes on when I don't need it to.
As I said in the OP, if it is dry... OFF
If it is anything else... wet/damp/uneven surface/over cast/chance of damp... ON.
Those are my rules, I will stick to them