With a algae scrubber, it's possible to maintain zero nutrients. My algae just sucks it up as soon as it's produced and aside from trace elements, I don't really have to do any water changes. The algae scrubber also outcompetes algae in the display, since I have periotic "black outs", I do this every 2 or 3 months, when I notice some pest algae take hold in some tiny crack or crevices. The roller mat (or any other mechanical filtration) has no method of nutrient export, once it's in its elemental form, dissolved in water, and is therefore dependent on water changes. Yes mechanical filtration will physically "remove and discard" debris and waste, but while it's in there, the waste is constantly in a state of decomposition, and in theory, be impossible to prevent leaching into the water column. The roller does try to solve this by "rolling" the waste out continuously, though there is still "leach" time.
The algae scrubber's "leach" time would be.. I guess if I didn't harvest it and it starts to die out, but again, that's simply resolved by the next harvest.
The scrubber also has "down time" if all the algae was harvested at once, however I get around that by harvesting only half of the mass, and alternating between halves, twice as often, which for me is rather than once a week, but every 4 days.
Another bonus, I can feed as much food, as often as I want. I simply get a more luscious, thick algae growth of ulva, which I feed a portion back into the tank. I use an upflow algae scrubber, incorporated into my overflow.
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