"Mining a full hex cost lots of ammo", yes, but no.
An hex is about 200km, but the scale is "stupid" in many ways, and "nice" in other ways.
Sure you can avoid a mine field in a 200km large space, but you can also avoid a 300 Gunmen troops then, or even a 10K men group with vehicles, actually.
But gameplay-side, no, you cannot. If there is even jut one small unit, you can’t avoid it.
So, gameplay-side, for me it would make sense to have minefields, bunkers, barbed-wire, and other fixed defensive structures.
I’d quite like having something similar to an "anti-road" (barbed-wire-like), to at least guard some borders from being taken too easily by anything walking in.