This advice hurts. Faerie fire improves more than hex does at level 5+ even if you gain multiple attacks. Why? Because all your allies are gaining multiple attacks too and more damage on such attacks, the spell itself is harder to save against, enemies are more likely to live a few turns longer, etc. The only situation that is not true is if said allies already have a relatively easy way of gaining advantage.
***Note: all advise does somewhat depend on other party members. Their abilities and their tendancies.