I'm a mature student who was also a student a long time ago. In 1982, I sat and passed the Law Society's professional qualifying exam. In those days, there was no such thing as an assignment.
The post says:
"They are nothing to do with ability". Oh yes they are. You won't pass the exam without the ability. Law exams, particularly, contain problem questions which you need to solve. Of course, you also need knowledge to draw upon to solve those questions.
That kind of skill applies to many other professions and skills.
Fast forward to now. I am studying for a BSC in Computing and IT. I am in my first year. Thus far, I have not sat an exam. The pass mark is only 40%.
How difficult is computing compared to law? Well the trouble is, I haven't been properly tested in computing. I have been given a ver easy ride. You see, to do well for an assignment, all I have to do is read the relevant text or article(s), formulate an answer then write it down.
Assignments only test your understanding. They do not test retained knowledge. That is why there is not a substitute for an exam.
The overwhelming majority of students can pass an exam if they put in the work for it. Impediments, such as slow writing are just an excuse for not working to correct the problem. If you are not capable, you do not deserve to be awarded a qualification. If you pass an exam, you are more likely to retain knowledge than if you don't. Furthermore, being able to pass an exam is an important ability in its own right. It tests your speed of thought, which is a vital skill in the workplace related to certain qualifications.
One day, a machine might be invented which can analyse the knowledge and skill in your brain but thankfully, it won't happen whilst I am alive. Until that time, there is no substitute for exams.