Can any developer comment on Source, Unity, and Unreal and how well they handle mods after the fact? Modding in PC gaming adds such an incredible level of valu

Can any developer comment on Source, Unity, and Unreal and how well they handle mods after the fact? Modding in PC gaming adds such an incredible level of value and creativeness that it being a constant afterthought is bordering on criminal. Steam Workshop is an interesting mechanic, but it would be nice if an engine set out a baseline in advance that didn't require using additional services.