Flashbacks were the subject of a course that I am taking on fictional writing.
I have used them in the things that I have written but I admit, I use them sparingly. They can be a great tool but they can also make the story unwatchable or unreadable.
Case in point.
I had to stop watching the show because every show has an incredible amount of flashbacks in it.
It is a great example of what not to do. At least, in my opinion.
The show could have been greater without it.