Tunneling rules work fairly easy and I have a lot of experience playing against them.  As long as they don't come with extras (like a breach charge that explodes and hits everyone on a AP5+/AT5+ within 15cm or other such nonsense) they aren't overpowered.
The nice thing about the tunnelling mechanics is that they are already established.
E&C, my question to you is are you planning on making the Hades Breaching Drill a unit, a war engine, or an upgrade?
Making them a war engine helps with the transport aspects but leaves you with the need for a model and unit stats.  Making them a unit forces the drill to remain with the formation and also leaves you with the need for a model and unit stats.  Making them a formation upgrade is the simplest solution, but leaves you with no models to play with (unless you just want to create one for the fun of it to mark where you breached the surface).
EDIT: ANy of these three will work just fine with the existing examples of tunneling in the only two lists that currently use the rules. The only thing I think that is NECESSARY is to make sure to work with those Champs if you think the rules need to be modified. I don't think they need changes, but I'm not in your head either (at least I don't think I am
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Here are the classic rules from Armageddon as a reference:This special rule can be used for tunnellers if
they are included in a scenario. Tunnellers are
set up on their own table edge before the battle
starts, at the same time that spacecraft are
deployed (see 4.0). Any units transported in the
tunneller are kept off table until it appears.
Write down the location where the tunneller
will surface at the same time and in the same
manner that you record the co-ordinates of a
drop zone (see 4.3.3). You must also secretly
record when the tunneller will surface. If it is
going to appear in your half of the table it may
arrive from the secondnd turn onwards. If it is
going to appear in the opposing half of the
table, it may arrive from turn three onwards. Set
up the tunneller at the start of the stated turn. It
and any units being transported may take an
action on the turn they appear.
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author of Syncing Forward and other stories...It's a dog-eat-dog world, and I've got my Milkbone underwear on.
