The change to Fel Armor is a BUFF in PvE and a nerf in PvP.
This is because as ur dps goes up so does the heals from fel armor and iirc it's around 17k~ dps to get to the same state the old fel armor was with 2/2 of Demonic Aegis (50/100% increase to fel armors healing making it 6% of all dmg done now or 4% total hp per 5 secs before) in both scenarios.
As you breach 17k dps in pve it becomes a buff for pve.
However, in PvP it is a downright nerf as there is simply no way a warlock will breach 17k dps in pvp.
It doesn't affect 3v3 arena or 5v5 at all neither does it affect large-scale pvp as a warlock should always switch over to demon armor when being nuked for extra armor and healing taken, but it is a significant nerf in 1v1 scenarios, soloing pve and 2v2 arena. This is because it was a reliable un-stoppable if u will passive self-heal. The difference is definetaly noticable. My locks unbuffed hp right now is 126k, with the old fel armor this would be roughly 5k healing every 5 seconds equating to 1k hp/s. To achieve the same hp/s one would now have to do 16.67k dps which is practically impossible outwith raid situations.
As of now I have no idea why they made this change as the only things it does is nerf warlocks survivability 1v1, 2v2 and while soloing.