I understand "no badges" at least in theory - because there are those who take the attitude "if I can't have what that person earned, then he shouldn't have it either." I completely fail to agree with it, but I understand it as a concept.
But why not costumes?
For the same reason no badges. It's not about fairness, it would be about prevent bugs/issues.
My idea is based on information from Codewalker talking on the subject.
Sentinel+ only records what the client had access to, which doesn't include a lot of server side flags.
Does character have X costume pieces? Y/N, and so forth.
Badges are more complicated. Having the badge to access Ouroboros wouldn't give you access to the Citadel, the flag does. But the flag is triggered by getting the badge. If you can't get the badge because you already have it...Then you can never gain access to Ouroboros.
Unless I misunderstood something, Which is possible since I stopped paying too much attention after Codewalker said "Not going to work."