It's hard to say just what effect the studio had on New Mutants. It seems they made Boone tone down the horror at first then did a 180 after IT and wanted more horror from the reshoots, but couldn't get the cast together so what was originally shot was what we got.
I think it's clear from New Mutants that Boone is a bad writer and director, in fact his writing would be bad by fanfiction standards. The film is a collection of scenes and not an actual story, because there's no cause and effect. Things happen for no reason and are promptly forgotten about when the scene changes. Even most crappy movies at least get that right. There are so many things that a competent director or writer would know not to do, like having this big reveal shot of the soul sword when that weapon's been used many times throughout the movie already. His dialogue was atrocious much of the time. And for someone who supposedly had a 'vision,' he lacked the minimal imagination necessary to figure out how to do Magik's origin right, show very little and play up the mystery. I'd say The Fault in Our Stars was a fluke that was only good because it was so directly adapted from the book and would never trust Boone to come up with an original film, story, dialogue, or characters again.
I can't fathom how the movie has any defenders. It's not like Mulan 2020 or Thor 2 or even Batman and Robin where there's at least a core story and a basic level of competence in the filmmaking even if the story or film is boring or trash. It's closer to The Room in how much it fails and the way it fails, only without being funny. Even ignoring what it does to the source material and the characters, it's a horrible film on pretty much every level.
When I did a rewrite of the movie, one of the things I did was try to restore the concept of cause and effect. So things like Roberto offering Sam a million dollars comes up again later in the film, and Roberto has reason to think Illyana would attack him in the pool and that incident changes how he treats her for the rest of the story instead of being forgotten.