Flashblacks and flashforwards in fiction - how useful/necessary are they? When does too much become

The use of flashbacks and flashforwards is a controversial subject among writers and writing advice pages. Some encourage flashbacks/flashforwards, while others encourage to avoid (especially if they bogg the narrative down or doesn't contribute anything to the overall plot). How does this criticism and in depth understanding of this literary device assist writers in improving their craft? How does this affect the way writers read/analyse flashbacks and flashforwards in fiction?

*Two novel's that could be discussed in detail is "A Visit from the Goon Squad" by Jennifer Egan and "Time's Arrow" by Martin Amis.