If Molten Man leaves the battlefield before his enters triggered ability resolves, the leaves-the-battlefield triggered ability will resolve first, and you will sacrifice a land before searching your library for a Mountain
Answering a creature or planeswalker costs around three mana so this exiles for one more mana - good at the kitchen table or Commander where you seek permanent answers in graveyard centric metagames
Sure, you can unearth the occasional Bad Moon, Gauntlet of Might, or Sunken City
Once you reach the phase of the game where you can afford to do multiple things per turn, you need card draw to ensure you keep having things to do (Circle of Power, Fight On!, Travel the Overworld) or at least cards which can be used multiple times (flashback spells, Ring of the Lucii, Shambling Cieth, Relentless X-ATM092)