The only problem i think is that you'll see them coming when they're digging so you'll be able to easily build in order to counter them, exactly like you see them coming now when there's a siege and so easily can counter them with well placed walls and traps.
I wish Toady would do it so you don't actually see the invaders (unless you have direct line of sights, or giving some ascii clues when your dwarves are succeeding Observer rolls vs Ambusher invading rolls to use the actual character skills) , a bit like ambushes worked in past versions , and so you can't see where they're actually digging and the direction they're coming from.
And even better if there's some "sound" clue to the digging so when a tunnel is starting to come close (like those sound clues you have in adventurer mode), your dwarves skills successfull rolls may allow them to actually point to the general direction of where it's coming from, or if they fail you can only read report strange noise in the earth.
Additionally as i imagine there are lots of people that worry about the tunneling ruining their fortress design, Toady should add an on/off option (like invaders are) , so people that don't want their fortress design being butchered will be able to keep at it, while people that want sieges being a real challenge will not have to face super toned down (and so too easy to counter) enemy digging.
In addition i hope invaders sieging will be able to build their own stairs and towers too, as i imagine an easy way to defeat siege (improved by digging functionality) would to build a fortress on top of some kind of tall pillars.
Would be great too to simulate siege towers or siege ladders too when you wall your fortress (as it already defeat completely current version sieges).