This is just you. I've used All Star, Soldier, Cactus, and Sunflower most of all (Mastered). All Star is great, just get better at aiming I'd suggest. Does more damage to the head. Soldier's are perfectly balanced. Cactus isn't soft nor an easy target most of the time. As for the new characters Super Brainz is intentionally slow, as is the Pirate so as to balance them. The Corn is slow and Citron is slow when he's not in ball form. Sunflowers are perfectly fine. Firstly they revive characters with full health, secondly they have rapid fire and with a damage upgrade they deal 10 damage per hit, and with a reload upgrade in addition to that they're fantastic.
I disagree with Super Brainz, he's perfectly balanced having to trade speed for power. Just don't let them get close to you.