We could make veterancy less cancerous if we removed the "current hit points" gain when a unit gains veterancy. The unit's max HP could go up, while the current HP stays the same. That would completely remove the whole "vet vet vet vet vet vet vet" angle to the game, which Thomas correctly describes as an annoying mechanic. ("You try to kill some unit and then it gets some more health and then you get mad.")
OR, the unit's current HP increases proportionally in the same amount as the unit's max HP goes up. So if an ACU has 1400 hp out of 10k max, and gets a vet, and goes up to 11k max hp, it would only gain 140 current hp (up to 1540). So the veterancy bonus itself would not provide an instant "get out of jail free" card.