The Nymph can only really have Rage Potion used on it once. And it's not usually worth it, considering how powerful it is as CC and that Rage Potion just makes it more brittle, and considering how expensive a 7|1 creature for 13
is.
Berserker allows the player to choose between a sturdy midrange attacker and a powerful, brittle attacker, as well as being quite cheap. Plus, because it belongs to Earth, it can be used much more effectively with Heavy Armor.
In my opinion it's not very similar to Red Nymph at all, apart from the Rage Potion aspect of it. They are used for completely different things.