I think the biggest hangup people are having is over the word, mitigation.
To me, mitigation prevents or lessens incoming damage.
Defense prevents damage from happening.
Resistance reduces what damage you take.
Slows and -recharge reduces the frequency of incoming damage.
Absorb shields you from damage.
This is mitigation.
Heals and regen do not mitigate damage. They react to it.
Now it becomes a matter of whether you can react to it in time and/or recover from it.
If Regen cannot, then there is a problem.
For regen to work yiu need a really high regen rate.
For that to work, you need to be a big sack of hit points.
When that doesn't work, you need to be quick with the heals abd hope its enough.
For that to work, you need a crap ton of recharge so that you will have a heal ready.
For that to work, you need a crap ton of -recharge resistance so you recharge doesn't get floored.
It seems to me a lot if things that have to be carefully planned for and go right just to get Regen to work as well as other armor sets.