"Classical but efficacious"
Translation:Classique mais efficace
Efficient and effective are not synonyms, though they often go together. Something is effective if it does what is intended to do, particularly if it does it very well. Efficiency is about minimizing some sort of required resource(s) (effort, energy, time, raw material). A fire truck might be an effective means of getting to work, but it would by no means be efficient.
Does efficace encompass both meanings, or just the one?
Efficace is primarily used to qualify something that was done (very) well.
Efficacité (fem) both means efficacy and efficiency: efficacité énergétique, coût-efficacité (cost effectiveness)...
Efficient/Efficience (fem) correspond to efficient/efficiency but are more rarely used, mainly by some professions: "améliorer l'efficience opérationnelle et de réduire les charges d'exploitation" = improve operational efficiencies and decrease operational costs).
Effectif/ve = actual
"Efficacious" is quite unusual, but I find the use of the word "classical" to be the real problem here. Perhaps this is a Britticism, but in American English, we generally reserve the word "classical" for a genre of music or a period in Greco-Roman history. Yes, one could say, "classical, but effective" if one really wanted to calm the crying baby with some Pink Floyd, but she only likes Mozart, but it's much more likely that we're talking in a more general sense about a time-honored technique that unexpectedly works as well as a new one: "Wow! She cut that long grass with the scythe faster than the guy with the string trimmer!" — "Classic, but effective."