Ok. This is interesting to me. Because yeah, the slow in MudPots on a Scrapper is so ineffective that I ran around checking everything in range with PA out of a suspicion it was outright broken. It isn't, it's just ... weak.
I tried out an Ice Blast sentinel to see if I could leverage slows. Nope. Not even a little bit.
But it's my Fire/Martial blaster with Reaction Time that is the only one that feels effective in slowing enemies, but that is because the debuff on that aura is 70% compared to much lower values in Ice Storm/Blizzard/etc. It probably helps that I run Gravitic on that blaster, which stacks onto the slow. Between that, RoF, and RT, this blaster is better at slow than every Ice character I've tried. Caveat: These are all DPS classes.