I found a post about hate generation, no clue if accurate but it makes sense to me, see below

" Shields only give buff threat (~70 threat), whereas heals give 50% of the total heal as threat.
Medica for 600 each = 300x8 = 2400
Medica II for 400 each = 200x8 + 70x8 buff + 100x8 per tick regen = 2960
You can see how WHM AoE gets ridiculous threat when used, but it does also correspond to the usefulness of the
heal. The threat from medica/medica II comes out to 4/5 300dmg/double threat attacks by a tank in threat (~5seconds of attacks?), which is why it pulls aggro so quick. Not to mention that the regen will generate another 800
threat every 3 seconds, which is getting close to the tanks threat generation as it is."

I am not an expert in all this, just trying to enjoy the game and keep everyone alive I hope this can help and please comment on this stoneskin thing^^
Cheers
KE