have seen them on mountain bikes for the wheels and bottom brackets but they havent seemed to catch on, i have heard of issues relating to reliability but this was when they first came out and i have no personal experiance of them.
as for there application to motorcycles have never thought about ti really.
I have noticed that the seals that fit after the bearing
have considerable resistance more so than the bearing.
So I have flicked them out, still have the bearing seal.
But I only ride on dry road, would not do that on a dirt bike.
Don't know about this brittleness you speak of. I know they are lighter and smoother and application is just a matter of cost. Worth putting in turbines etc but a set for your bike would be I am guessing $400 or so. Drag racing friends were picking them up from Boeing surplus around here.