The skeleton units are a nice touch!  I'm not sure if you're looking for feedback, but I have a suggestion for a new unit type that might help achieve a better balance across the various unit types -- 

Hot air balloons!  Basically the Militant Vegans (4 HP, 3 Move, relatively low power), but they can move across any terrain type at a cost of 1 point per tile, like an airplane.

I find that I rarely build the tank factory and instead just save up to build the hanger.  The unrestricted movement of the fighter jet and bomber is a bit overpowered. 

One reason for this is that the player's city is often sandwiched between mountains and shoreline, with only a single narrow path in/out.  As a result, all units with 3 or fewer movement (infantry, skeleton, both tanks, militant vegans, snowman) end up following the same path and not encountering any new special units to recruit along the way, which is key to victory.

I'm not sure if you would want to make it a special unit or something you can unlock through the city, but I think a hot air balloon or similar unit could help correct for the power imbalance between units.