I've seen plenty of people shame others about this when it's not really explained.
Effuse by itself is not a good single target heal because its ratio of mana:healing is simply very inefficient. That said, it can be considered in your healing priority as a last resort if all others are going to overheal. Two situations pop into my head:
- You're healing the tank with ReM; EvM + SooM and it's not enough. Also, nobody else in the group needs healing. You can spam Effuse.
- One member of the group got hit with something and they need to be topped off before a nuclear boss ability. E.g. Shadow Bolt Volley from the last boss of Black Rook Hold will one shot your group in your high keystone and everyone has to pop personals, so they should be topped off.
This is only even acceptable I would say in a Mythic+ environment since there are no other healers. However, Vivify does
barely a little more healing than Effuse so some prefer to spam Vivify in situation 1 above with the benefit of healing other members if they spontaneously take damage in that time frame. Having infinite mana in a Mythic+ also doesn't help Effuse's case. Another obvious flaw is even the slow Sheilun's would take priority over Effuse if you're confident you can get the cast off in time. Properly anticipating incoming damage will let you use the rest of your toolkit to heal with greater efficiency in general which is why people call Effuse a noob trap.
In a raid, by comparison, Effuse is only going to deplete your mana when another healer more tailored to single target healing will do it much more efficiently. Mistweaver in a raid setting is punished severely for improper mana management. You can use Effuse in a raid if you get the +200% buff from the legendary pants, though. I would also say if your raid lacks a holy priest or a holy paladin using Effuse to heal the damage from the random eye beam in Mythic Iggy is not incorrect due to how easily that damage can double dip on a player and kill them.