The sensitivity is the most important factor, a high impedance pair of headphones can be easier to drive than a pair with low impedance if the sensitivity is high enough. Very few IEMs need more power than the average phone headphone socket puts out, I can only think of a handful, the Final e5000, Tin P2 are among them. I don't know why so many people only consider the impedance though, some headphone reviewers and retailers also overlook the sensitivity and a few only list the impedance in the specs.

IEMs almost always have very low impedance so you need only refer to the sensitivity.