The only reason gamebryo modding is "easy" is because the community has been working on tools for it for decades.

It uses weird proprietary formats nobody else is using and full of stupid quirks. Back when Morrowind released, officially all Bethesda provided along the construction set was their plugin for a thousand-dollar licenced product to make nif models. Nowadays they don't even do that anymore because they know people have been making their own free tools for Blender etc.

As for the modding structure itself, other games support a plugin hierarchy like they do. Rimworld in particular, and it runs on Unity.