As noted here a primary function of pretests is to cause anxiety and depression in the instructor, especially if no remedies exist for the problems identified.
If you don't have the ability to enforce the makeup they indicate, or even to get the students to drop out, they do little good except make you wonder about the common sense and motivation of your students.
Nonetheless they do give you useful information on what to review.
Once in the distant past, puzzled as to why my own dept had abandoned this seemingly logical process, I gave one the first day of calculus.
The average score out of 100 on a test about equations for straight lines and area of circles, and definition of sin and cos, was 10 except for a 100 from one non native Chinese woman.
I gave the tests back graded and told them to work them out open book and I would re grade them. Several students just did not come back, and of those who came back the average score the second time on the same test, open book over 24 hours, was 15.
I just gave up the idea ever since that. The information they give is something I already know: that most of my students will not have the prerequisites for the course, any course, ever. I still have to teach it, so i always include a review of everything they will need. Half of them still drop out or get less than a C, how could they do otherwise?
However, i still recommend some kind of pretest because it helps you get to know your class, the main requisite for you to teach them. But a less traumatic such measure may be preferable such as learning their names and asking them questions about prereqs in class in an unthreatening way.