Stability? A BDFL can make decisions at a whim that radically destabilize and change the project. Lording over and causing immense harm to the community without any sort of accountability is a recipe for disaster.
Good for everyone? A BDFL can unilaterally make decisions that prioritize their own desires over everyone else. It becomes about their personality rather than the project.
Functional? A BDFL is less functional, seeing how there is no requirement that the BDFL do anything that the community of people actually wants.
If you're saying you're opposed to democracy because you don't like the decision that codeberg reached democratically, the BDFL can do the exact same thing. Except the process is far worse, because there is literally no input from anyone else on making the decision.