So I took a look at the Bonelab SDK 
Working with avatars is pretty easy, most of the work is done outside of the unity engine so the rest of the work is just importing and making sure things work right (which most modders don't seem to care to do, almost every quest avatar has broken thumbs)
A large problem, however, is the level editor. SL0 left out access to a ton of features that seem pretty necessary to ease of use. I haven't found any assets to generate something like a "Return to Hub" button easily. Doing things in a way the SDK isn't specifically scripted for also causes issues which are hard to diagnose until you realize you were supposed to do the same thing you just did, but in another way. Normally this wouldn't be an issue, but SL0's documentation is barebones at best. I suspect this is the main reason for most mod levels just being map ports, it's relatively easy to import a model, add colliders, and shit it out on mod(.)io. Unity addons seem to work well for doing things outside the SDK's scope, but going down that rabbit hole creates a lot of unknown variables that might not work well with the marrow engine. 
I would say that much like the campaign, the SDK lacks depth. SL0 seriously needs to push out updates to add functionality to the level SDK before pushing out other features as unprepared as this.