I used lull/pacify a lot when leveling my cleric from 1-70. I still use placate in PoFire when pulling named from around guards. So long as I stayed inside the level ranges capable for the spell I had no real issues. Also as long as I never cast a second paci type spell on a mob that had resisted the first I never got aggroed. If the mob resisted and I tried a second cast I would always get attacked.
I use the spell line a lot on both my cleric and chanter, if there was a request to be made on this, I would ask for chanters and cleric to get one that worked on mobs up to level 72-73, but it should be noted that would not make them solo better

For zones I know placate works in Seb, Icewell, Chardok Royals area, PoValor, and I imagine anywhere where mob level does not exceed level 70.
Placate is different from Wake of Tranquility (level 55 ae paci) and Pacification (level 65 single target paci). Placate only effects reaction radius, and the other 2 spells effect reaction radius and frenzy radius (not sure what frenzy radius does).