I get the idea to try to enhance immersion, or build up tension and etc., but I see a problem for me as a DM to implement this idea, be fair to my players and don't feel dumb myself. The game already has a lot of asymmetry on availability of information between players and the DM. The proposed variant goes one step further, as now the players don't have information on their own HPs, but I as a DM have information on the monsters HPs. Every time the players use healing it will be an extra gamble (rolling dice + blurred info on current status). For me to use resources healing the monsters it will never be so blurred. So, to be fair, what should I do? Make deliberate suboptimal choices for the monsters to balance the ground? Just play the monsters in the normal way I would but balance down encounters a little bit if I want to preserve the original challenge level? None of these seem good enough for me. So, something different?
If somebody knows how to handle this, please share your methods and techniques.
I am not good at pretending not to have information that I have. This is even the reason I decided, when DMing, to start rolling dice in the open, so to let the players make decisions better informed. I feel this enhances their tactical approach to battles, with their reactions and such. I also make some effort in-game to let the PCs access information on monster weaknesses, specially the ones I know the players have from their long experience in the hobby. I encourage them to evaluate monsters capabilities by describing their equipment, emphasizing physical and behavioral characteristics, so they can judge monster defenses and try to play their offences accordingly, etc. I also feel this way I am free to build up the challenges in interesting ways for my players to interact with, as the less information there is, the more the game feels like just gambling to me.