This is my personal opinion on the relative comparison of different APIs, frameworks and interfaces used in this project by order of different metrics.
Usability by developer from most to least
1. Console: its just standard input and output, can't be simpler
2. SDL2: biased as I'm confortable with it
3. XForms: very simple and painless
4. Qt5: painless
5. OpenGL 2.x: easy but have to adapt to the state machine
6. XLib: its a bit special
7. gtk3: it should have been easier, but hell
8. WIn32 GDI: in itself ok, but the very weird dirty microsoft quirks ruin it
9. httpd: a lot of spaguetti code for adhoc HTTP but straightforward
10. fbdev: very simple but also bare, sometimes buggy?
11. OpenGL 3.3: writing shaders...
11. motif: simple but broken and weird behaviour
12. wayland: it just sucks, and its broken
13. xcb: useless documentation and unexpected behaviour (might be me)
14. Vulkan: a thousend lines for the initialization, and bugs probably my fault
15. Win32 ComCtl: jesus christ how does this still exist