“No Effect” vs. “Little Effect”: A common mistake is using 无济于事 when an action has a small effect, but not zero. 无济于事 implies a complete lack of results. If you mean “the effect was minimal,” it's better to use a phrase like
事倍功半 (shì bèi gōng bàn) - twice the effort for half the result, or `收效甚微 (shōuxiào shènwēi)` - the result was negligible.