If you have veterancy based on proportional damage dealt, then it's a single step away from an EXP system such as you have in other games, such as Etherium. The only difference is that the experience is granted on kill, and not progressively as damage is done. Implementing that would also use less memory, since you can update veterancy status as damage is dealt, instead of storing it until the kill is actually scored.
Also, what happens with regen and repairs when you are recording damage for veterancy? Could you end up scoring double from a brick that had been in a few fights and only died later? How would overkill work? Will Percivals score 'bonus' veterancy from one-shotting T1 tanks? Similar problems might arise with overcharge.
As an extreme case, if one player had a single brick, next to a large number of hives and had a donated paragon somewhere, and their opponent had a monkeylord lasering it for ages, would the monkeylord snap up to full vet when the brick finally died?
Also, what happens if an almost-dead experimental is self-destructed moments before being killed. Does veterancy for damage even get scored?
Personally I am fond of SupCom's kill-based veterancy, because it sets it apart from other games with arcane exp system. I also like following hero units around and seeing them be amazing.