I have another point of view, but it’s very subjective.
I have used Grease Pencil and Opentoonz so far, and this provided me headache and frustration for different reasons in each. Mainly because I can’t dive into the Blender logics for 2D, Opentoonz does not work proprely in Linux anyway.
Synfig is the only one I don’t struggle with. I don’t know if it’s Linux-like or Inkscape-like logics, but I feel at home when having to undertsand sthg new.
For sure, no popular ‘drag-and-drop’ UI concept, but anything customizable with values, this is very intuitive and quite simple.
Of course, some things might be improved as in any project, which is a question of time and contribution as ever.