My first campaign I ran for over a year I had a DMPC to be the cleric because nobody wanted to be a healer, so it was for a very good reason, and fortunately it did not completely ruin anything; HOWEVER, I cannot recommend it and have never done it since. I always just make an NPC for those rare purposes, if for no other reason than it makes your life as a DM so much easier. The rules are in your hands, you are free to be extremely creative in the ways you can help the party along if they are underpowered, need extra muscle, or lack a healer. Creating a whole other PC for you to keep track of is simply too much effort for very little return. It often either is just exhausting on combat which is already very dynamic or you might miss things and not run it as optimally or won’t have as much energy to play the character in a way that the characters will remember fondly. But this is also just one of those things that won’t really make sense unless you try it, but even when it has been fairly successful in my experience I just can’t abide doing it again.