Beneficial bacteria will thrive in the canister filter. But only aerobic bacteria which produce nitrates.if u are planning on keeping fish only in a salt water tank then it is no different from a freshwater tank. Nitrates will build up till u do water change And fish can tolerate high nitrate in the water.
The problem comes when u keep corals especially Sps which will not tolerate high nitrates and phosphate.
In order to remove nitrates and phosphate u will need some different sets of equipments/reactors/skimmers and all the other unsightly gadgets. So where do u place them ... In a sump...
Then If u ask why can't a canister filter remove nitrates and phosphate .. They can't cos the water flow in a canister filter is too fast and too oxygenated for anaerobic bacteria to trive.