I've driven in some very heavy rain storms and was impressed by how well it keeps track of everything. However, in those heavy storms, I still drive manually. It's not for fear of the AP system losing track of anything, it's the risk of hydroplaning. Being dead center in a lane isn't always the best place to be. But if the road is clear of standing water, then I have no issues letting the AP system do the work.
I've also noticed how well it can still see when the wipers aren't auto activated. Visibility will be very poor, but it still sees everything. I usually have to manually trigger them because I'm not comfortable being in the car with such poor forward visibility. It's my theory on why auto wipers are so terrible. The system simply thinks visibility is still "good enough".