That's a good point. If this module goes in the MPU slot, you can't take the MPU that refabricates drone parts/ammo, so the limiting resource in a dive could become drones left. (I do not think this module should go in a hardpoint slot for this reason, but am not sure how all the rules for what slots can do what are implemented in the game.)
But you are right that this module does a lot of things in a single slot: it does the work of tug/haul nanodrones (a hardpoint slot) and an MPU. So it should consume resources rapidly.
I think if I purchased this module and had the base number of drones (1,000 kg), I would hope to get a near full hold of one type of processed ore. To start thinking about, I think a conservative rate would be 1,000 kg of drones per 5,000 kg of final, fully processed ore.
For larger drone containers, I think this rate is alright. If you take 5,000 kg of drones into the rings, you get 25,000 kg of processed ore. This doesn't sound like a large amount to me. But if you think it is too high, I would emphasize that if you can't reclaim remass, you'll have to be conservative with your maneuvering to find good ores, or go to a Phage-class station to refuel (expensively).
Currently, you can take the top-tier MPU that refabricates drones/ammo/remass and stay in the rings (almost) indefinitely, with the added bonus of being able to have small drone and remass tanks to reduce ship mass. In my opinion, no remass reclaim on an MPU is a severe drawback.
I believe 25,000 kg of processed ore is about a quarter the capacity of a Titan (including its detachable cargo pods)? I think this is reasonable: even upgrading to the 10,000 kg drone container does not allow you to stuff a Titan with this "drone MPU" at this rate.
Overall I was thinking this kind of MPU would exist in order to allow ships without cargo holds to exist, which could allow for all sorts of cool looking designs. It is a way for ships to have less of a mining focused loadout but still pull in some income to stay afloat, in case an anomaly hunting, exploration ship fails to find anything.