is it possible for the work permit to be rejected ?
Yes, it is possible that your prospective employer will not be granted permission to hire you, in which case you are plum out of luck, and there isn't anything you can do.
what do you think the percentage of not completing the process ?
This isn't a casino. It's real life. And any "percentages" would be completely irrelevant to your particular case. It is enough to know that it could happen, it does happen, and it might happen to you. So don't make any plans to travel or quit your job until your prospective employer has ALL the ducks neatly lined up and you are 100% sure they do indeed have a job waiting for you.