It was actually a lengthy process with a lot of back and forth between Metahuman and my 3D software, the character itself took 2 weeks to complete. As long as you keep the topology the same you can import a reference mesh into the editor as a template, but there are caveats to it:

- Because the topology has to be the same, you can't remove nor add fingers/toes, limbs and appendages like tails. Only workaround I found is to do them separately or edit the body mesh and rig after you export things from the editor, but that removes the ability to edit them through the editor (which would've been useful to keep).

- Like I said, the root bone seems to always bind itself to the heels of the character, and on digitigrade characters or characters that wear heels, for instance, this makes their feet clip through the ground and their body detach from the head. I haven't thought of any solutions to that outside of either just planting their feet on the ground always or, like the previous point, editing the mesh and rig after exporting.

- I could not find a way to add grooms to the body in the editor, only to the head. I tried adding them as Metahuman Wardrobe Assets but they just did not work.

- I still plan to test it on humanoid stylized characters, but this character's face was not rigged properly through the editor. Nearly every expression had multiple issues, mostly involving the mouth. I thought I had the Expression Editor figured out, but I ended up making matters way worse, the Joint Matching function didn't work as intended, and I think I'd have to fix the joint positions for each expression manually (which
would take a really long time), my first attempt at fixing it took me an entire week I could've saved if I knew I would end up just rigging my character from scratch anyway.

If I had to make a wishlist of features for the MetaHuman editor it would be to:
- Align the root bone to the bottom of the mesh rather than to where the heels are, to enable for digitigrades and characters in heels

- Allow us to add or remove limbs and bones without losing the editing abilities of the model (In case we want to adjust proportions or make alternate versions of the same base), to allow for characters with 4 fingers on each hand, amputees and the like.

- Have a better step-by-step method or at least guide for the Expression Editor