I don't feel the Twilight Cleric is "broken". Game breaking is usually when one character takes up too much of the spotlight, like dealing the most damage or making other characters feel irrelevant.
The Twilight Cleric is strong, but in the right way- they make their allies stronger, as opposed to, say, a 3.5 Clericzilla, using Divinely Persisted buffs to make themselves more powerful in melee combat than the Fighter or Barbarian, while still able to cast magical spells if they choose.
If enemies focus fire, a lot of those temporary hit points being handed out to other party members don't matter as much either. Again, not discounting the fact that a Twilight Cleric makes encounters much easier, but at the end of the day, it's basically just preventing damage.
It's not making fights quicker or letting a Rogue routinely one-shot your major enemies.