First of all, Undertale was made in Game Maker... main issue with GM is that the code is interpreted, so it's slow. (Unity and Godot compiles stuff into C++/C#) But you can definitely make complex stuff in Game Maker, especially if you're doing a 2D game (GM's 3D in particular is notoriously slow, and it has no innate 3D model editor or even WYSIWYG level editor)
I can vouch for Godot too, it's specifically made to be easy to use for people that aren't programmers, and after sifting through some basic tutorials I can kinda agree with that. Unity is getting a bad reputation for creating asset flips these days, and they recently did an out-of-nowhere EULA change that screwed over a lot of people. I'd avoid that if I were you, even if it's got a lot of support.