The thing is that a Tank can only agro what they hit, and there’s a 6-mobs-per-hit limit, whereas a Healer puts out a “Hey! This person is doing a thing!” ‘aura’ which draws a little bit of attention from any and all mobs. The agro generated by a heal is less than what is generated by a hit from a DPS or Tank, but if there’s a lot of mobs they’ll all agro immediately to anyone doing heals first and only peal off if someone else hits them.
That’d be why some Tanks prefer no one else heals them, so that they’re the only one putting out that ‘aura’ and making it easier for them to grab initial agro from mob swarms.
If there’s just one boss in an encounter the Healer is gonna be the lowest on the agro-meter, but yeah, having a Healer on the team makes it harder for a Tank to grab the agro from all of the mobs when there are swarm mechanics, and with so many cleave/AoE attacks from bosses it’s not a good idea to just have a healer stand with a Tank to have all the mobs go to one spot, because the Healer is not gonna last long in the line of fire.
Now I haven’t personally noticed any differences with Unveil Essence, but it is possible that, since it’s applying a leech effect to the target, and anyone hitting that target gets heals applied, it might be dispersing that ‘aura’ out to everyone instead of just the Healer, which can make rounding up mobs even MORE difficult for the Tank, but that’s just guesswork on my part. I find that I prefer Healing with Fists (even though I have, and use, Blood and AR on occasion), so my experiences with UE are limited to only certain specific encounters in the game.