In my experience they can reduce algae-related maintenance, but they won’t eliminate it. Mine won’t touch black beard algae, spot algae, or the thick fuzzy green algae that used to coat my driftwood. (My nerite snails eat all of the above, though BBA is not their favorite. Very happy with them.)
Bristlenoses do, however, keep non-spot algae off the glass pretty well. I used to have to clean algae off the glass every few weeks. Now there’s just spot algae to remove every so often.