"El a fost eficace nepromițând nimic!"
Translation:He was effective not promising anything!
Both are grammatically possible. In this sentence, the adjective "effective" modifies the subject, "He" (referring to how effective he was), whereas use of the adverb "effectively" would modify the verb "was not promising," essentially referring to the effectiveness of that action.
Are you saying that “El a fost nepromițând nimic”, without the adverb, is a correct sentence? Like “a fi nepromițând” = “being not promising?”
So what does this mean, exactly? That he succeeded in not promising anything, or what?