...is that there are no type specimens or field markings to positively identify them. There are basic guidelines for identification--the general shape, altitude, and opacity of the cloud--but only sometimes does a description or photograph of a cloud in a field guide really match the cloud you are studying in the real world.
Take the photograph above, for instance, taken by my niece in Washington, DC. Judging by their layered, clumpy form, shading, and estimated altitude (low), these beautiful clouds are, I believe, stratocumulus perlucidus clouds. Stratocumulus are one of my favorite types as they usually move with some urgency across the sky almost like a time-lapse video. And we have plenty of them here in the Pacific Northwest. Here are some other examples of stratocumulus perlucidus clouds.
You can see that they are all generally similar--low clouds which, according to Michael Allaby's Encyclopedia of Weather and Climate, appear in "patches, sheets, or layers of gray, white, or both gray and white cloud. There are always dark areas, shaped as rolls or rounded masses. These sometimes merge into larger masses."
But stratocumulus always seem to be changing from or to another type of cloud such as stratus, cumulus, or altocumulus, so you'll see characteristics of these clouds in the stratocumulus. That makes them hard to i.d. But that is part of their charm. Just go with "stratocumulus-ish" or "stratocumulus-esque" and you'll be fine...ish.