hallo, thank you all for the explanations!

Throbert, 

 "ты слишком мягко сказал!" can be used technically, but in most cases people would say: Это ещё мягко сказано! (impersonal structure is common in such cases), (meaning literally, 'this sounds even mild/understated').

Another Russian equivalent which, i think, is most close to 'You can say that again' is "Не то слово!" (то is stressed here), literally 'This word is not enough to express the case'