If the carriage and drilling units are representative (which is how we have them now), we get complaints about the models themselves. Why bother having them? IMO the easiest solution is to have them representative as it speeds up the game.
we already have precedence in this area with spacecraft. they are onboard / table edge, etc as a model but representative of a force ability. "Why do we have models for them then" is not a reasonable argument (Argumentum ad lapidem I border on but ...
IMO the best way forward is to not have the Termites/Moles/Hellbores count as anything other than cover once they surface.
I agree OR they can be a separate formation that works along the lines of droppods do (again, existing precedence) to represent their supporting fire abilities (if any/appropriate for the unit - Hellbore I look at you)
Carriages or no carriages: Should the carriages that carry the Termites/Moles/Hellbores be allowed on the board at the beginning of the turn? After all, there are models and we should use them.
Let's be honest. People have models. They are justifiably proud of them. They want to see their pretties played with. I can dig it [cyrus impression]. However as they worked in SM2E is fundamentally at odds with the abstracted nature of 4th Edition and so we must flip the viewpoint a bit I think and everyone can get what they want.
First of all we have units that can carry other units. Some lists require them to be part of the formation, others do not. e.g. Rhinos vs Thunderhawks. While this tends to be the case with WE not being part of the formation (Stormlords aside) there's no hard fast rule with that. This is part of list construction and therefore if an issue can be handled via unit stats and lists instead of special rules, all the better. The point is that units carrying other units that in of themselves are units is not something disallowed, just weird. The various proposed stats for Cap Imperialis for example work like that with their ability to carry AV, which in turn could be chimeras! I should also point out Hellbore and Cap Imp are both the SM2E Praetorian models produced.
The problem that comes up is the practical application of using them as units. Are they War Engines? Armored Vehicles?
Give the the tunneler stats. Give the carrier stats. Some will be AV. Some will be WE.
Does the drilling piece and the carriage count as one unit or two for the sake of targeting?
I advocate 2> keep reading.
If it is two, it creates all sorts of messes: which one gets struck first, can the drilling piece launch without the carriage, what if it is targeted by artillery?
Make them two separate formations that a list design then makes a fixed compulsory amount to take. Carrier + exactly 1 tunneler.
If it is a single unit, the obvious drawback is that your tunnelers can be targeted before they launch. Would a Squat player ever want to take that chance? If so, at what point cost?
They should be valid targets but if they're separate formations then why even target them? Treat them as allowing a scatter reroll in the list IF present on the board. The player then can drop them (similar to transport) but they then lose the ability to better target the breaching location. They've all launched prior to the first turn (call it turn 0) but the force then decides to advance providing "spotter" support and can be a target. Perhaps that's not a risk they want to take but then they're considered in the rear baggage area and effectively out of game.
Now I have a reason to take my pretties and show them off. The opponent has a reason to target them. Also gives me a reason to leave them behind. Seems pretty fair and balanced. Special Rules are totally kept to the list level and do not change core global rules.