I think it depends on the situation. Every adaptation is different and presents its own unique set of challenges. IMO, though, typically the closer the screenwriter sticks to the source material, the better the final film will be. Of course there are always exceptions. Imagine how bad Jaws would've been if the screenwriters had stuck closely to Benchley's novel.
A year or so ago, someone was selling what they claimed to be a copy of Alexandro Jodorowsky's 1976 script for Dune. Jodorowsky claimed it was fake on Twitter "Es un falso guión con un dibujo idiota." ("It is a fake script with an idiot drawing.") It still ended up selling for $6000
However since then, the people at DuneInfo have read this script and compared it the Moebius' storyboards for the whole film (as seen in the documentary) plus Jodorowsky's own artwork and believe it to actually be genuine.
Even if you still doubt the script's provenance, in this article you can still see Moebius' storyboards (which include dialogue) for pretty much the entire film and get a pretty good idea of how it would have turned out. with its orgies, mass-defecation, death by crystallization/deadly birds impregnation by blood, fixation with Crucifixion, and action scenes that wouldn't be out of place in a spaghetti western.
So if you're interested give it a read/look, as this is probably the closest most of us will get to seeing the Jodorowsky screenplay
Interestingly from a screenwriting perspective, while the script adds many bizarre elements, it also makes some things clearer than many other adaptations of Dune. Leto gives a much clearer reason for why they're going to Dune despite knowing it's a trap, we see more of him trying to prepare for the inevitable betrayal, Yueh does more to help Paul & Jessica, Duncan Idaho's and Kynes' importance is shown more clearly.
Don't get me wrong, I still think if this film had been made it would've been another Zardoz (and it still rushes the 3rd act) but it's not as utterly outrageous as I'd been led to believe
I learnt that the hard way, parsing through hours and hours of email attachments when it first leaked. There were several detailed (page) breakdowns and “notes” on the drafts he’d done which were pretty enlightening. Hoping with talks of Denis Villeneuve talking over as director on Cleo, rewrites will result in Roth’s leaking