Depends. I have had an alloy nipple fail, but I suspect that was in part due to the tyre sealant which had got into the rim. Generally not an issue in my experience. And I've never found any difference between good alloy nipples and brass in that respect.
It's a fairly small sample size, but I've never had any problems with Sapim Polyax nipples. I have had a fair few issue with both DT Swiss and Sapim brass nipples, especially the black ones. Polyax is my default choice of spoke nipple these days.
Depends. I have had an alloy nipple fail, but I suspect that was in part due to the tyre sealant which had got into the rim. Generally not an issue in my experience. And I've never found any difference between good alloy nipples and brass in that respect.