The 50% slow on hammer is only 3 seconds. So it's fine.
Regulas Mark of the Betrayer I believe is 30/40/50% slow. I think it only lasts 5 seconds though. This is fine too, since it's one of Regulas currently least desired skills, and takes 3 points for low duration high slow. This change will sort of buff that skill, make it more desirable.
Guild Wars has lots of caps on things too, but there is an exception when one single skill does more than the cap.
Like there is a cap on "reduced healing" at 33%. You can't stack "enemy takes % less healing" more than 33%. But there is an elite skill that drops it to 50% less.
It's actually better that way because it makes those few skills that do more than the cap a lot more special and useful over stacking two small ones, since stacking the two smaller bonuses will hit the cap and do much less than their combined amount. This would be great in Demigod, even though it's a very different game. =]
I have to reference GW a lot, because they did so many things perfectly, and it's an online RPG like Demigod with balanced teams, not an mmo and such.