Yeah, the "this thing isn't broken, because the DM can fix it" is a terrible argument, and also contradictory. Why the heck would a DM need to fix a game mechanic if it isn't broken? I've never changed anything in my games that I thought was completely fine the way it was. I've never changed Paladins, I've never changed Barbarians, and I've never changed Warforged. However, I have changed Dragonborn, Monks, Rangers, and Yuan-Ti Purebloods before (to make Dragonborn stronger, Monks less boring, Rangers actually useful, and Yuan-Ti not absolutely broken). "If it ain't broke, don't fix it."
Why would we want to fix this thing if it wasn't broken? I know what I've seen, and in my campaign, the Twilight Cleric has been OP as %#$&. They've regularly out-damaged the TCoE Beast-Master Ranger with a Longbow, the Archery Fighting Style, and Sharpshooter, and have been able to solo kill/survive tons of monsters that should have been impossible for them to take on at that level. And not only is it OP, like the others have said, it's remarkably boring. It's spamming the same 3-4ish spells/abilities over and over all the freaking time, and I'm saying this as someone that has played a Warlock. It's just overflowing with super-useful abilities, and none of them encourage creative play, it's just "activate this, attack that, boom! They're dead and you're immortal".
I'm a powergamer. I like having powerful characters, and as a DM I like it when my players are powerful heroes that face huge challenges. I like my games being on hard-mode, and the Twilight Cleric just makes it easy mode way too often.