Obviously it will better fit Pharo since its made to work with smalltalk code, but that does not make it any less terrible. Just because you have one implementation of something that does not mean its good. Its just means its there and it works.
I dont know the internal, they are not documented anyway, there are some class comments here and there but thats pretty much it. I dont even remember when was the last time monticello got an updated, I mean a serious update not just a couple of bug fixes the 2 years I have been around.
Secondly GUI is just plain awful, Smalltalk maybe be the first or one of the first to implement guis, but those implementations never ended up to something that would be approachable and easy to use on a day to day basis, some tools suffer more from this some less, Monticello is up there with the worse design.
Thirdly the inability of the system to version control images , audio files and other assets it defeats the central purpose of smalltalk of everything being objects with a loud "Nope !" from Monticello "Only source code is".