As 3dees stated, they can be used but they need to be cleaned more often than most filtration equipment. In reef tanks they are better used as a reactor for Carbon or GFO.
Lots of reefers use them. Most won't admit to it though. Even a sump/fuge can become a nitrate factory if not properly maintained. Eheim & Fluval make the best of the best canister filters. They cost but you should never skimp on a canister filter.
That is true of all filtration equipment....the difference is though that the sump is much easier to keep clean....everything is visible and the amount of detritus can monitored.