I think that the problem doesn’t lie in how much functionality does an in-game editor has, but how easy is for players to access these editors. I think that most modders won’t mind to have an Epic Games account.
is what Epic could do: create a plugin, that allows players to create an Epic Games account while inside the game, without the need for the launcher. After the modders create an account or login, the game downloads the required files for the in-game editor.