In Supcom/FA campaign, you are a fairly new recruit, and you don't have access to all the tech immediately.
We can assume that battles are only fought with ACUs, and they are rather expensive to make, and we can also assume that they don't give one to any soldier.
In GW, you start as "any soldier".
So the question is : Do they give a state of the art ACU to a noob ?
I don't think so.
The idea is that you don't have a full ACU at rank 1.
Rank 1 : You start with a ACU that does not have access to any upgrade, overcharge and with a smaller blast radius. It's visually the SCU model. The max health is also reduced to 75%.
Rank 2 : You have access to overcharge (still SCU model). 80% health.
Rank 3 : You have access to all tech suites. Full health.
Rank 4 : You have access to a real ACU model, with all gun upgrades (including torpedoes)
Rank 5 : You have access to all upgrades.
Rank 6 : Starting from there, you can buy heath & regeneration upgrades (limited by rank) up to 150%.
Each rank give you a bigger blast radius/damage, with a really big one for rank 7/8 (maybe even bigger than a nuke visually).
In parallel :
Rank 1 to 3 don't have access to quantum gateways or experimentals.
You start with a smaller unit limit (from 250 to 1000 at rank 5). Probably more cosmetic than restrictive.
Gameplay wise, it make ranking up even more important. It also make suicidal players rather inefficients.
Of course, ranking up from 1 to 2 will be cheaper enough to be rather fast.
Thoughts ?
