Thoroughly amateur opinion here (so take it with the requisite grains of salt): I think that flashbacks are often the tool of lazy writers, standing in for proper characterization, foreshadowing, and dialogue in order to hammer home some necessary background detail, character trait, or “prophecy” (and not just the cheesy fantasy sort).
Rather than restricting the characters to the arc of time encompassed by the story proper–and laying out everything the reader needs to know about them with more “sophisticated” tools–the lazy writer might instead just opt to craft some scene from whole cloth that will justify a character’s behavior, a situation the plot has wound its way into, or similar.
For instance, take about 50-70% of the flashbacks in the TV show The Arrow. Here, we flash back to when the titular Arrow, Oliver Queen, was “trapped on a hellish island for five years” and witness him–in rough chronological order along those five years as the series has progressed–learning some valuable life skill, problem-solving technique, or combat art that is immediately and exclusively relevant to the plot at hand. And until we reach the 90% mark in the episode and “finish” the flashback scenes, Oliver won’t use that skill/technique/art, even though he desperately needs to. . . and more often than not, these things never come up again.
Rather than having a character who’s displayed a trait–say, the ability to resist poisons as needed–organically through the series, the show just waits until Oliver’s forced to fight a poisoner-foe, and we as viewers are helpfully transported back to a point theoretically exactly five years previous where he learns that skill, and when we flash-forward again, he’s remembered! Voila, poisoner-villain defeated, and skill lost forevermore.
Now, all that said, I don’t think that flashbacks are automatically and immediately bad, depending on how you define the term, really. A story that takes place at multiple time-scales–Steven Erikson’s Malazan Book of the Fallen series comes to mind–can be a totally legitimate and interesting thing. Playing with time and place can be a great way to make use of an unreliable narrator, or to achieve some desired literary effect, or to maintain a mystery/withhold a clue.
Hell, in a series like iZombie, flashbacks are more or less the meat-and-potatoes of the storytelling, as the zombified mortuary worker absorbs the memories of the dead in order to solve crimes. The flashing back is a core character trait, and the entirety of some plots will hinge on the clever use of flashbacks–especially as sources of incomplete, misleading, or otherwise frustrating material.
Indeed, a flashback to something as ambiguous and open-to-interpretation as the story’s “present” can be very effective, especially when compared to the all-too-convenient “just what we needed to know” variety of flashback. It can serve to heighten tension or cast new light on already-confused situations, both of which could be really interesting and thoroughly not-hacky.
And now I’ve written far too much about flashbacks. You know, that reminds me of when
<cue Family guy flashback gag>