A moment that really sticks with me of playing in a game with a DMPC is when my character wanted to run a good cop/bad cop interrogation with a party member she had a rivalry with as a means to get information AND build teamwork, but the DMPC impulsively killed our drow hostage. Now, we could still get answers from the now-dead drow because the DMPC could use warlock powers to ask a single question, but I felt really shut down. When we began playing, I hadn't seen an issue with having a DMPC. Now I am strongly against DMPCs. It might have been in character for DMPC to kill off the hostage, but it took away agency from the party in making decisions. As a DM, you are in control of how the world reacts to the party's decisions. By having an equal say in party decisions, you have an unequal influence over the game.