I think there's a compromise here. I suggest "tunnel through anything" but have it be skill dependent. A novice miner can eventually hack his way through granite (one of the hardest known RL rock types), but never leaves behind a boulder for later use (or non-use) as he's had to chip away the offending rock splinter by splinter.
Digging dirt should rarely, if ever, grant exp. It's just too soft, right now a novice miner can dig through it like, well, soft butter (you there, yes you, grab that shovel and dig me a hole! Now! Is it easy? I bet not--I got tired digging 1 foot by 1 foot holes for plants). If dirt gets harder (novice miners have trouble) then exp, but again, amount of exp based on skill. If you're a grand master miner digging dirt is like drinking booze, you shouldn't be getting exp (or at least, not much).
Now, this'll be annoying because people want only their high skilled miners digging at the tough stuff as to Get It Done, and there's currently no way to decide who mines what, it just happens. Now, in my last game (HangingGardens) I regulated my mining industry HEAVILY, but the completion of the lower portion of the Gardens I had four miners, only four, ALL LEGENDARY. I actually went and de-miner-ified three of them because my Professional Fourth was losing ore (precious, precious coal) and I didn't want to micromanage, so I turned off everyone else and told this guy to mine exploratory shafts until he was blinking legendary, then turned the other three back on and set to work.
Four got the job done and never lost a stone, gem, or ore. So in my suggestion, if you only use your highly trained miners (and train up one or two beyond your starters) and kill/reprofession the rest you'll never have a problem with "novice miner takes forever finishing the one last tile in the room I need done!" There's really no need for an army of miners anyway, four legendaries get the job done FASTER than twelve (legendary or otherwise), as the other ten who went on break claim a square and have to walk all the way across the map instead of letting those guys already down there to do it after atomizing the rest of the room.