one of the advantages of riding on rollers is how you can fine tune your effort.
take this ~35' interval
almost constant speed (34,4km/h) and cadence (82), with the expected increase on HR (med.131bpm). It's not important the average values nor the relative effort but how constant they are for all that period.
When running, or even riding outside, is much more dificult to keep that within the narrow chosen margins.