Modding APIs also bring downsides by being a complex subsystem of the project for the maintainers to deal with, taking away time that could be used to solve other problems in the software,
but it also increases the capabilities of the project by approximately ∞%!
and (in my experience) they often lead to fracturing of a community and too many options to choose from.
this is good! i like having choices. you can always choose not to use mods