I think the associated "bug" with Pride's Fall supports the case that this behavior is not intended.
If the unity follower combo is not a bug, then every single person in the game needs this. Doesn't that seem like an unintentional consequence? Why would this combination trump every item in the game?
It's intentionnal and made to work that way.
Even if you take more damage than your follower when he doesn't have the legendary, you don't take more damage than you would if you had it.
That's because followers are supposed to take less damage than you do all the time (since it's easier than having a smart AI to make then stay out of damaging areas).
If all your party members had a unity, it would be a flat 75% damage mitigation before your own mitigation kicks in (which is why it's either useless or godlike depending on how many people have it).
The bug is that with the follower legendary you take no damage when your follower does nor do they, the other is that on T6 your follower will die instantly and thus you should start taking full damage, which is why it's a flat damage 50% reduction because it's always the same damage on all damage you take.
With 4 real players the damage is not a flat 75% reduction, if everyone is taking damage its a 0% reduction and the damage you take changes based on who is taking damage unless you run with 4 players that all run the same class with all the same gear and 2 stand off screen so they never take damage then you can t replicate what happens with follower plus legendary
We'll just have to agree to disagree.