Having recently read a few books who use this device, I realized something.
It ruins stories.
I'm not talking about characters speculating about future events based on known facts here, I'm talking about mystical future sight that presents the far future as (possible) fact.
Now I admit, there are multiple ways to go about weaving a prophecy into your fantasy novel, but most of them have problems. Let's take a look at the possible variants first.
The 100% accurate
This is the most straightforward version and not often encountered. The problems with this variant should be obvious. A character prophesying something that later comes true exactly as predicted is the same as writing a spoiler for your own novel. It kills expectation, has the plot between the prediction and the event predicted locked in place. The how isn't compelling anymore because the reader knows where it's going to end up. It completely strips character agency leading to a dull read.
The 100% accurate, but not like you think
This is a variant you see often used. Prophecies are always true, but ambiguous enough to be twisted around to make the reader's assumption wrong.
This one can go three ways. Either its so ambiguous that the predictive value is non-existent; the event the reader (and frequently the characters) believe to be described and work to counter/achieve is actually not the prophesied event; or it falls somewhere in between.
In the first case, why even bother? If a prediction is so ambiguous that the reader won't realize it's fulfilled until after it happened, what's the point of writing one to begin with? Neither the reader, nor the characters can do something with it, which means the author might as well omit it entirely.
In the second case, you end up with a similar problem as for a completely accurate prediction. It completely strips character agency, with the difference that the reader isn't aware of this until said prediction comes to pass. This sort of plot device is a major downer, because it paints the character(s) that tried to prevent the prophecy as powerless afterwards. The reader is left feeling like they've been getting invested into something that was always going to end as it did.
The possible future
The variant most often used in novels. A future is predicted, but with the immediate note that it can be changed. This type can be a powerful plot device as the characters strive to prevent/achieve it.
Unfortunately it's also a lazy one. While it allows the author to conjure a new problem for the characters to tackle, the problem presented is 100% plot generated. Worse, it's 100% plot generated from a plot that hasn't even has come to pass yet. This, again, strips characters of agency, although indirectly this time as the prophecy is simply thrust upon them.
The provided motivation is always suspect. After all, if a prophecy isn't an iron clad certainty, why should the characters bother with it? It might not happen anyway. Similarly, if the expectation is that it will happen for certain, why struggle?
First off, it is very possible to make it work and many novels do just that, by tweaking reader expectations and making the predictions appear as a natural part of the story.
Unfortunately, a lot of the times the author doesn't go that extra mile, using prophecy as a crutch instead. Leading to above mentioned problems.
My advice to authors would be: avoid using this unless you are certain you can work it in a way that avoids taking away character agency, without becoming superfluous.