how upgrade works is on vet level, they get extra 20% hp + attack of the base level. on guard level they receive another 30% as well as the initial 20%, but both based on the base stats, so guard effectively gives you +50% overall.
then you have royal guard, which gives you extra 10%.
now that is 160% of the base stats, or 133% of the vet level(160/120).
ok here is where cuirs dont follow this rule -- they dont have a sub-vet base stats!! their base stats is vet stats!!
what does it mean? it means their royal guard upgrade actually gives them 40% on top of the vet level, where most royal guard only gives 33% on top of the vet level.
age3 cuirs: 500 hp, 30 att
age3 vet mus: 180 hp 15 melee
age4 royal cuirs: 700 hp, 42 att
age4 royal musk: 240hp, 20 melee
dont think 7% is nothing, 7% is winning or losing, it means they last 7% longer + they kill you 7% faster.
you may say is it because they are age3+ only unit?
Maybe yes? halb gets the same system, (180hp in age3, 233 hp in age4 normal guard) but halbs are not OP.
And no! because port goons are not on that system, port goons are upgraded on base hp of 200(age2), not 240(age3), even they are age3 unit and can only be received in age2 by a card which NO ONE actually uses.
then again cassadore is on age3-stats-being-base-rate system , yet you can get them in age2 if you allied with french and he used team age2 skirm card.
i think this is something definitely needs a fix.
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update: the gap widens even further in imperial.
i posted this here by request of ceres.