They don't entirely suck - it's more of a balance issue currently. Not enough people are playing Citron or Chomper in pubs to deal with the Super Brainz/Imp spam in some games. Corn spam works a bit on defense, but he's a bit too slow moving to be great at offense.
I will agree most of the cacti variants feel clunkier than in the first game, but I still do great currently with the Zen and Future Cactus.
The main problem with playing cacti now is that you're usually getting cornered by imps, all stars, or superheroes at close range. Two of which get 200 hp baseline - the Imp can get into Z Mechs which can have up to double that. (You only had to deal with the all star in GW1) Frankly, it's not super fun to empty your clip into spammed bullet sponges over and over if you're playing offense. (When you should be focusing the 125 hp or less classes)
TLDR Changes I'd like to see:
-Corn variants get a more respectable move speed than the turtle crawl they have now.
-Buff some of the Cacti variants ranged damage and/or increase the damage modifier at closer ranges so skilled cacti can hold their own in a pinch.
-Increase Citron Beam duration or damage. Buff the shield slightly? Buff the Electro Citron's uncharged shots. (4 damage is NOT okay if you need to spam shots)
-Decrease Super Brainz's health to 175 or less so he's more in line with the chomper. NERF the target arcing on Electro Brainz's beam.