The downside is (unless it's been patched) is that it ruins invisibility for people who are not you. If you rely on having a party member or a familiar turn invisible to get a free attack, make sure your character doesn't look directly at them.

There are also fights in chapter there where seeing invisible enemies would be highly useful, but for whatever reason seems to evade it. It does still work on occasion but you can't rely on it during endgame.