That, I agree, is very true.
I recall the horrible stress and anxiety I experienced early on in high school when I ran out of time in a mathematics exam and thus couldn't complete all of it. Panic started to set in because I faced the dilemma of having to decide which of the questions I could best answer (and that was far from being clear). The alternative option was to sketch out parts of more of the questions and leave each partially unanswered (on grounds of obtaining some marks fór at least having some knowledge of the material).
I cannot remember exactly what I did but I think it was a mixture of those two approaches. What I vividly do recall however was that I wrote a somewhat sarcastic comment on top of my examination paper with words to the effect that the examination was too long to do within the allotted time.
Under the circumstance, that wasn't the wisest of moves, it didn't win me any Brownie points with the teacher. Also, I was ridiculed and humiliated by my classmates as they laughed and crackled at me when the teacher stopped to read my comment out during his handing out of the marked papers.
I can't remember what marks I received except that it was borderline fail (and I can't even remember what side of the line the marks were on). I do recall the only consolation I had was that some kids received lower marks in the examination than I did.
There are two reasons for why that exam remains particularly memorable, the first is that it was first [but not the last] time I'd done badly in what I considered an important and key subject (unlike languages which back then I considered of little importance—much to my later chagrin and regret); and second, I knew that I could have done much better if I had had more time to compete it. Still that's hardly a reasonable excuse for being poorly prepared (which was a fact).
Here, I'd also make a distinction between examinations where one simply doesn't know answers to questions and those where one cannot provide satisfactory or adequate answers in the time available (for whatever the reasons), as marks won't necessarily provide an accurate assessment of a student's actual knowledge of a subject.
Again, that's unlikely to be a problem if one is sufficiently prepared. Nevertheless, there are some students who are particularly adept at doing well in exams and I'm quite envious of them or their ability. Especially, so those who've less interest in a subject than me and or who have demonstrated by other means that they've less knowledge of it than I have.
So be it, the world's not fair, and neither are examinations the ideal way to determine one's knowledge—but likely they're still the best method we have for said purpose.