First of all, player-managers, when they play, should focus solely on the game - and not individual players - to be effective, so they shouldn't be able to know exactly how well individual players are performing or even how well their plan is working in different parts of the pitch. On which basis, then, does a player-manager make substitutions?
Secondly, in order to substitute someone on or off, they need to give a signal to the assistant referee (or, more likely, to the assistant coach) so that they know who is being substituted. However, depending on the player-manager's position, they could be far away from the both the assistant referee and the assistant coach, and it's unlikely that they would be able to afford to temporarily leave their position just to make a substitution. Therefore, question two: how can a player-manager give the right signal for substitution?
And, finally, there is of course going to be bias in that the player-manager probably won't want to sub themselves off, and that they will be more likely to sub off players whose decisions they don't like (e.g. a player didn't pass to them). And so, final question: how do they deal with that bias.