Ancalagon
Dusty Dragon
I both really agree and disagree with you.It’s clearly broken. There’s no argument there. And it’s so bizarrely broken. D&D5E fights are already super easy mode. Most fights are a cakewalk unless the DM goes out of her way to unbalance things. It takes some wild dog-piling to get things to the point where the PCs are even challenged, to say nothing of in proper jeopardy.
But I honestly wonder: so what? Most groups have to bribe someone to play the cleric. Having such an obviously broken subclass (well, two with the Peace cleric) will hopefully draw some munchkins to the role and help ease the shortage of cleric players. If it’s such a pain in play, just talk to the players and ban the class. You don’t have to allow it at your table. Saying no to your players about busted stuff doesn’t make you a bad DM.
A more balanced approach, like in 4E, that allows the healer to both heal and do damage regularly might help. Players don’t want to be a heal-bot. It’s boring.
It is broken and in such an obvious way. If it had been a single "burst" of temp HP it would have been pretty good! But with it lasting so long... so yeah, I don't understand why they did this.
However, I don't agree with your notion that "in 4e you could heal and damage" because you totally can in 5e, and especially with this subclass. A twilight cleric can use their temp HP aura to "heal" the party while unleashing a barrage of offensive clerical magic - spirit guardian, spiritual weapon etc etc.