From experience, riding rollers regularly will makes you more efficient on an actual bike, because you have to keep the bike relatively still under you on rollers in order to stay upright.
You don't have to keep the bike that still under you on the road, and that wastes energy.
You can quickly tell if someone is a strong cyclist by looking at how straight they go, both hands, one hand and no hands on the handlebars. All over the road? Usually weak. Can stay on a road line? Probably ok. Can stay on the inside edge of a road line? Look out.
You don't have to keep the bike that still under you on the road, and that wastes energy.