Identifying a bad advisor (before applying)
There is probably no issue here and the careers of his students are determined primarily by their actions, not his. It might be a function of the popularity of the subfield in which they work.
But if the professor is listed in the Mathematics Genealogy Project you can find the titles of the dissertations of the students and check for yourself whether their work was significant as students. You can also find out whether they &quot;produced&quot; any mathematical heirs.