I like boxed text when it is 3 to 5 sentences that gets the point across. Otherwise, it's too long and the players are likely to forget everything by the time I'm done reading it. I've seen old adventures where the boxed text runs longer than a full page which is just nuts.
Count me stridently opposed. 3-5 sentences is least terrible, but you have maybe 30 seconds before you need to introduce a decision point to the players.
If you plan on writing boxed text, here’s my unsolicited advice - establish scene and tone ASAP (use few, heavy, visual words). Next, Call out important features, NPCs, interactive stuff. Finally drop a decision point that invites players to take some kind of action.
Since you need to do this at the beginning of every scene, and often as the entirety of a transitional scene, it is best to dispense with it quickly and efficiently. Save exposition to reveal during play as part of interactive scenes or when recalling lore (and even then keep it tight).
To date I have not used any boxed text in a game as it’s been written (well, since becoming an adult anyway).
P.S. on backstory. If backstory was ever any good, it would be main story. The less explained it is, the broader the strokes, the better it is. Obviously this is just my opinion, but think of the last prequel you saw and ask yourself how it held up to whatever the main story was. There are very, very few prequels worth their own full treatment.