To my understanding, a digital miner can mine any kind of blocks even if:

• block has high hardness like an Reinforced stone or equivalent,
• block is a tile entity like a machine or sort,
• block is part of a force field,
• block is non-accessible (behind a wall of unbreakable blocks like bedrock or one of the previous).

Could we have at least an hardness cap or a blacklist to address this?

In 99% of cases, people in need of serious restriction of the DM would be better off using a miner from another mod- it's fairly easy to disable Mekanism machinery. Regardless, for in-game limitation of the miner, check out the currently-open DM overhaul thread.