Like you say the problem with EO's as with any natural cosmetic ingredients is that there is quite a margin in the variation of its composition. With EO's that can be even greater as a variation in composition means a variation in scent. Other issues with EO's, especially those sold in small quantities, is that there is a much bigger chance of adulteration (for example mixing geraniol & citronellol and sell it as geranium EO). Good quality control is costly, skimming on that means skimming on quality. However scientists often publish their results of GC/MS analysis of EO's, so you can scholar.google "GC/MS analysis xxx' of the EO if your choice and get a rough idea of what it should be.
P.s. to clarify GC/MS = Gas chromatography–mass spectrometry
Here is a good article on the matter: