I can say for sure that opening a file read/write from res:// will fail on some platforms and installations.
And save files only in
res:// is for program resources and is often located in locations where the operating system does not allow writing.
It is better to discover sooner than later that you should not write to
res:// unless you code some plugin/addon which is used from within the godot IDE during development.
I can attest that godot allows writing to
user:// also from singletons because I personally use functions in my
global.gd to write savegames to