For me i think it’s much easier to do a workout on a turbo, no traffic lights or junctions and you can maintain a constant pace. This also means it’s a bit more tiring than the road for me, and also time passes slower without anything whizzing by.
Mile for mile its just as effective as the road i think, a lot more efficient for planned workouts i’d say, because of no traffic or sharp turns etc.
Time is a better measurement for the turbo though. Some people say it takes away from your bike skills but as long as you ride a real bike every so often i don’t think it makes a difference. Hard to say how many real miles compare to turbo miles, if you have a long straight road with no interruptions then they’re probably similar, but then you have the wind and road surface to consider.