Is it harder than Strange Journey, Etrian Odyssey, or Dark Spire? That's a tall order.
Depends how you term 'harder'. SJ, EO and Dark Spire are hard because of specific encounters that either require specific tactics (or party builds) or grinding to higher levels. They're also hard because in the case of SJ and EO you have a 'survival' element where half the tension involves making it back to the save point.
With Grimrock you can save anywhere / anytime so the difficulty comes from the situational encounters and the frantic nature of battle. It's tile-based real-time so you'll open a gate then find yourself in a room facing a group of enemies that are weak on their own but become formiddable if they surround you or attack from range while you're already engaged in combat. It's also 50% a puzzle game with each floor giving you multiple logic challenges to overcome before you can progress so difficulty also comes from working these out.
For oldschool CRPG'ers like myself it's a blast, and possibily easier because we know how to use the mechanics (enemy movement, hit and move, puzzle types etc.) so I'm not finding it hard - more like a fair challenge.