You don't use them because they are new. You use them when they offer a better solution to the particular problem you need to solve at the time.
You need to be able to get confident that the manufacturer will keep on shipping them, but rejecting a part just because it is single sourced isn't necessarily wise.If you really need a particular part you can sometimes buy a life-time supply.
MOSFets and GaN Fets look like fairly safe choices, not that I'm too well-informed these days.