Both Duo and SpanishDict say eficaz can mean either effective or efficient. "Simple, pero eficaz" is a statement out of context, but "simple" could imply less waste, and for that reason I chose the translation, "Simple, but efficient", and was counted wrong. I am reporting it. Both effective and efficient have the Latin root efficere, but efficient potentially can connote less waste. They are listed as synonyms in dictionaries.
The DL hint for eficaz shows both effective and efficient as being valid.
In English, this sentence would work with either word. Granted, they're two different ideas - which would totally depend on the context of what you were trying to confer.
DL marks "efficient" as wrong, even though there is no context to this sentence.
I deliberately translated the sentence as 'simple but effectual' in the expectation thst Duo would reject it. It did. I wonder if English is the only language to have such similar words as 'effective' and 'effectual' with very slightly different meanings, which vary according to which usage book one consults.