It’s certainly possible to use these as NPCs or any other character. It just takes a bit of work, you aren’t necessarily going to be doing it out of the box.
There are 2 things you can do with Metahumans:
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Use the head only, attaching it to a body mesh you created.
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Use both the head and provided body. Build your armor/clothes, etc. around that base body to match the proportions and default skeleton.
For #1 you’ll need to have a bit of experience with removing a head and rigging it to a separate body. Ideally you’ll want something to obfuscate the fact that the body/head are separate. This is generally done through upper body clothes/armor of some kind. If you’re going to be showing more skin, then some sort of neck guard/choker/ornament to hide the break.
Other than that make sure to retain the blendspace data in your modeling program of choice (i.e. In Maya do not delete the historty) and simply rig it, weight it to the skeleton, and you’re all set.
The blendshapes will still match the necessary requirements to work with LiveLink and you can animate the character in your preferred modeling package and bring it into UE4 to polish it up with facial capture. I don’t know if MetaHuman comes with any control rigs for the body, but they’re not too difficult to setup.
In the case of #2, you’ll simply model your clothes/armor around the existing body that is exported with the MetaHuman. That way you’ll retain the rig proportions as well as a seamless neck/body transition that will prevent the need to cover that area. You could also theoretically modify the shape of the body so long as you don’t disturb the area that connects to the head which could bring it out of alignment. Could be useful if you wanted to give your character more size or musculature for example.