The acute pain paper they cited (linked in other comment) said "low-quality evidence [...] for a small but significant reduction", which seems clear and correct to me. If these authors think that's too favorable, then the paper I linked above suggests "insufficient evidence to confirm or exclude an important difference".

Either of those distinguishes "strong evidence this doesn't work, and more studies are probably wasted effort" vs. "weak evidence, more studies required". I don't see any benefit to a single phrase covering both cases unless the goal is to deliberately mislead.