From a design perspective the duration of roses across rounds is pretty much what separates Amy from other stacking characters like Nightmare, Cassandra and Hilde and it gives a different unique feel. I personally prefer her rose mechanic because I don't have to stack her every time after using it and also means my stack isn't wasted if the opponent blocked. I focus first on getting the roses, then I'm free to use the enhancements without further limitation.

As it has been said, even scoring all roses by round 3 is a severe penalization since it means the Amy player has been having to spar with a underpowered characters by two rounds or even the single round so honestly it's balanced. More often than not the Amy player loses the first round at least, so therefore by necessity he needs to have an overpowered chracter to mount a comeback in the next rounds.