@PotomacBob
So, narrators who are 80 cannot devote a whole chapter to, say, their own sixth grade, even though it is important because of the influence it on on their later life. To me, that sounds like an unnecessary restriction. Why should that be a rule?
As everyone knows, there are no RULES in publishing. Whatever you can get away worth is considered legitimate, though traditional publishing houses would beg to differ. But I never intended to set limits on anyone. Instead, I'm merely advising newbie authors to initially watch their step. A chronological narrative (pick a point in time, then do mini flashbacks before returning to that timespan appears to be the easiest to carry off.
Also, as noted, certain books are simply more suited to flashbacks than others, though honestly, I can't formulate a way to list what makes it so. All I know is that, if the book is not suited for it, you're probably going to waste months trying to 'prop up' the multiple flashbacks and flash forwards, only to end with an unmanageable mess at the end (based on my own experiences, and the various books I've read which employ them).
As always, if you have a knack for flashbacks, then knock yourself out. Otherwise, your muses should seriously start waving warning flags when you first begin considering it. But, IMHO, limiting flashbacks to one consistent 'chronological narrative' seems like a handy, safe compromise. Again, there's no Rule against doing it, but they simply become more problematic if the timeline just jumps around, confusing the readers as to what's actually happening.
I've dumped a few books that've done that unsuccessfully, while I can count those who've done it successfully on one or possibly two hands.
P.S. Prologues and Epilogues are exempted, of course, since they typically take place outside the 'normal' scope of the story anyway.