Two of may favourite series of books have acquired spinoff prequels in recent months, Jack Campbell's 'Lost Fleet' and David Weber's 'Honorverse'. Both these prequels are set centuries before the main series and attempt to flesh out the history of their respective universes. The big difference is that where I really enjoyed Campbell's 'Genesis Fleet: Vanguard' I gave up on 'Manticore Ascendant: A Call to Duty'. There were a couple of minor reasons for my different reaction and one major reason. the minor ones were that unlike Lost Feet the main Honorverse series is still full of loose ends that need tying up and this is the third or fourth spinoff that's been launched, also in in contrast to previous Weber stories where the protagonists tend to be charismatic and super competent this time he, perhaps deliberately, has gone against type and created a protagonist who isn't at all charismatic, in fact a character I found positively aggravating
Neither of these things would have been a deal breaker, what made me give up was the book's continual attempts to build up dramatic tension around the fate of what amount to fixed points in the Honorverse, that is things which fans know full well are going to survive until the time of the main series books. it just struck such a false note with me that I couldn't finish the book. Now this isn't an issue specific to Manticore Ascendant, it seems to be a common problem with prequels, a genre which seems to be on the rise recently.
I had much the same problem with the Gotham TV series, the show set when Bruce Wayne is still young and centred around Detective James Gordon. It also insisted on cramming the cast full of characters from the future Batman canon. This meant that every time on placing some character in peril it's invariably one that we know is still around in the future and after a while it just becomes irritating.
Prequels don't have to be fall into this trap, I enjoyed the Genesis Fleet novel and Rogue One was a great movie. Both created an array of new characters and locations where the individual outcomes were unknown even if when you where it all ultimately leads. I suppose its the difference between real danger like skydiving versus the illusion of danger you get on a rollercoaster.
In essence I guess my view is that if you're going to create a prequel either avoid characters protected by the plot armour of future events or at the very least acknowledge their invulnerability and avoid putting them in phony peril.