This is often the big question from clients. Is an industry insider, reader, producer, or consultant able to look at a screenplay and guarantee its success?
Unfortunately, screenwriting isn't a concrete science. Screenwriting is an art. Watching movies is akin to a walk through an art gallery. Not everyone is going to enjoy the room exhibiting the abstract paintings.
While there are screenwriting success stories out there, it doesn't mean every single one can be predicted. Who knew that J.K. Rowling would be turned down by major book publishers or that George Lucas struggled to get Star Wars accepted?
What a screenwriting service can do for a screenwriter is better their chances and ensure that there aren't formatting errors or typos which would harm their chances of getting past a tired script reader. Further services looking at story and structure can rescue a script from boredom and provide that necessary second set of eyes to enhance the screenplay for success.
It's just that there simply is no scale of 1 to 100 and that a finished script can tick all the boxes and so assume a mathematical success. Hollywood goes through cyclical trends, be it genres or a run for dinosaur features. What might be turned down now may have significant draw and relevance in 2 or 3 years time.
The bottom line is getting your story completed to its best potential and out there. Don't let a reader make a call on whether your screenplay has guaranteed success or not. After all, you wouldn't take notes from a motorbike salesman about the chances of a new car being the best fit!