I do not doubt that that is your experience, but stating more concrete issues is more constructive.
Do you recall the last time undo did not work for you? There is currently an open issue regarding undo, maybe your issues with undo match that ones described there? That could help to reproduce it.
Do you mean Glyphs does not remember the copied value and thus you cannot pasted onto a layer in Edit View? Or from another application into Glyphs?
It’s difficult to give a good estimate for how long something will take to write, especially now where Glyphs 3 both added a lot of new features and also extended many existing features. Also, the type design landscape is quite different now since the last major revision of the handbook (e.g., variables fonts are now a thing and that affect almost every part of the handbook).
In the meantime, the tutorials and release announcements cover most of the new topics. Relevant tutorials:
Version announcements: