yeah, not really a good idea
It would be very hard to work how costs would be calculated appropriately. Eg what formulae to use? It would be way to complicated. Only because it would be very hard to come up with equations to determine the effect the unit would have on the various routes
I believe the potential for abuse is way too high. Im sure people could work out how to make their route invinvible ect.
So this leads to the most obvious problem, which has already been hinted at in my post. It would totally screw up route balance. Sometimes every few rounds a couple of units may get minor changes to help balance routes. This would totally undermine all that hard work and change the whole way the games works.
So this would make it much harder to predict smaller battles, and even large ones if these custom units were used significantly.
Even if you applied certain constraints for each route (i thought that might be a possibility, but...) it still has much potential to unbalance the game.
I probably couldve condensed my points more efficiently, but im a tad tired
Anyways, at least your making suggestions
[edit] lolz, seems a few people got in before me, sorry to pretty much repeat what has now been said xD