Feraligatr Prime did have the same weakness but it was around during the period where Yanmega Prime was still popular. Because of its weakness to grass, it could easily be 1 shot. Also, its primary attacker would have been Samurott (ability) who had a weakness to lightning energy, and to a lesser degree Kyurem NVI. Since Samurott was also a stage 3 (2 stage 3s make for a clunky deck) and was in the same set as with Zekrom (ZPS & Zeels were 2 of the more popular decks), it never took off. It wasn't a bad card, but the meta held it back.
Keldeo's ability lets you tank (and use max potion before you attach any energies to it) for your blastoise, keeping it on the bench (assuming its not 1HKOed), and overcomes it's high retreat cost. It's a beefy basic so it's a threat right away. And it has a relatively rare weakness (basically your only threats are Shaymin and Virizion). It also has the potential to 1HKO any pokemon (As far as I remember only Mewtwo and Rayquaza can do that..and Mew I guess)
8/10