Well, unless they can go through bedrock, then it comes down to, have an open area around your underground base (top to bedrock).Zombies dig, they come out in the empty area.
Where the player and defenses just smash them.
More work for the player, but really, negates diggers completely.
I'm currently clearing a HUGE area around my base (soo many goodies to mine it's just insane)
If I go 40 blocks out all the way around, let'em come. They''ll die.
Now, early on, they will be an issue. Putting the forges way out somewhere underground, will only work if they are under a drop pit.
(which, thinking about it, is what I should have done).
They find the heat, come look, fall in, and the turrets/traps etc take care of them.
So in future, it'll be base down to bedrock (with topside fortress), find good pit location at least 2 chunks away, dig pit to bedrock, put forges etc on bedrock, but above that, is the bait of open hole. Plop, zap, bam, loot.
So unless they start digging from far away, non-issue.
As someone else said though, how do you defend against them?
Open area before they get to the vitals.
Outer wall can just be wood. Cheap, easy to repair. Not as if they will really keep them out.
I do want the basher one though, that type should already be in the game. That at least makes sense.
just my 2 coppers, again.