Even on Amumu at 3,000 hp that's 35 healing over 2 seconds, most fights don't last more than 2 seconds these days while people have thousands of health, and most champions either have some sort of life sustain which is MUCH stronger, or even passive health regen that's stronger than that.
Keeping in mind this is Amumu, pure tank. So best case scenario. Demolish isn't in any way "resolve" and the shield one only works on very specific champions.
On top of that, aftershock can be 50/50 as it takes so long to trigger, especially early game ganks. The 2.5s defense is nice though, if not for that I wouldn't even primary resolve on Amumu.
I'd say they need to 3x Font of Life 1%->3% to make it feel okay even as a non keystone.
They should also consider adding another defensive thing there that's more generic to all champions, like increase armor/mres on boot items by 25-50% based on level.
There are so many options for damage right now, but riot is so highly resistant to defensive ones that even the defensive tree has a bunch of gimp masteries and offensive ones in it.