Flat betting goes soon to the house advantages. Some sort of variation in bets is very needed, and it has not to be a progression neg or pos , it can be other ways. Flat bet or triggers can only work if we for some reason know what the ball is up to or can conclude from history, which in my option we can not.
I hear very often "if you can not win flat you can not win other ways". That is often just a parrot talking, because the sender never explain why.
Roulette IS a game of chance, but it is possible to some extent manipulate that fact. Variation in bets is one way.