For example, user which could not change status or priority in forms should not do it, but for priority there is one way:

Steps to reproduce

1. Show the user only a form, which do not let him edit the priority (normaly he should not change it in a way)
2. Now login as the user, who should not change priority
3. He can not edit it at the form or as a action, but he can change it in another way:
4. Go to the workboard, where you can see the task
5. click "order by priority"
6. Now you can change the priority by drag and drop, maybe to all values, but a user who normaly should not change the priority can change it.