Actually you got it completely wrong man. Whether the dyno is a 'heartbreaker' or not depends on the calibration and have NOTHING at all to do with the 'load'. Even dyno figures from different operator running the same dyno can be different. That's why if you want to do a comparison, just stick to the same operator. If all you want to do is a power run to compare before and after mod power levels, any dyno would do, as long as it is the same dyno and operator, but if you were to go serious in a proper tune job with steady state and all, you would need a load bearing dyno.
Generally there are 2 type of dynos, Inertia dyno where it uses a fixed roller mass and calculates hp by the time taken for that fixed mass to spin up, or a load-bearing eddy-current dyno, where the energy is absorbed by a electromagnetic retarder and the energy produced and RPM is calculated to obtain the power figures. Generally inertia dynos have very good repeatability and can often be paired together with a retarder to get the best of both world. Dynojet is an inertial dyno in it's core but it can be paired with a retarder. Many cheaper dyno operators use the base Dynojet which does not have a retarder so tuning on that would not be able to give you a complete tune, or at least will require more time and effort as it cannot do steady state. GTAuto have 2 dynojets, one of them have a retarder while the other doesn't if I am not mistaken.
Mainline, Dyno Dynamics are pure eddy current dynos and adjusts the retarder load on the fly depending on the power to control the rate of acceleration. 'pure on road simulation' is probably just what the tuner tells you but Mainline ain't really more accurate in 'simulating on road conditions' than any other dyno with a retarder. Accuracy of the AFR tuning in the end depends on the tuner and how patient he is. Some tuners just run WOT tune and calls it a day, skipping the most important and most time consuming part-throttle tune.