Previously I wrote about the danger of comparing oneself to your peers. It can easily lead to discouragement, anxiety, and a loss of self-confidence.
It is also unhelpful for students and postdocs to compare their present advisor/supervisor/mentor to past advisors.
It is also unhelpful for a supervisor to compare their current students/postdocs to previous ones.
Early in his career Professor X had an absolutely brilliant student Y who made an important discovery. [Decades later X and Y shared a Nobel Prize for this discovery.] Apparently, X compared all his later students to Y, and could not understand why they could not be as good as Y. Sometimes he even let the students know this.
I have also known people who have had an exceptionally helpful undergrad/Ph.D advisor but then were always unhappy with their graduate/postdoc advisor because they just weren't as helpful.
Making comparisons is a natural human tendency. Sometimes I struggle with it. But, I try not to. It is only a route to frustration, to difficult relationships, and failed opportunities to help people develop.
Every individual has a different background, training, personality, gifting, and interests, leading to a diversity of strengths and weaknesses. Being a good student/postdoc involves innate intelligence, technical expertise, mathematical skills, computer programming, giving talks, getting along with others, writing, knowing the literature, time management, multi-tasking, planning, conceptual synthesis, understanding the big picture, being creative ....
Some of these will come very easily to some individuals. Some will really struggle.
The key is to accept each individual where they are at now and help them build on their strengths and realistically improve in their weak areas.
I think their are only two times when it is helpful and appropriate to make comparisons. Both should only be done in private. In writing letters of reference it may be appropriate and helpful to compare a student to previous ones. In deciding who to work with (e.g. for a postdoc) it is appropriate to research the advisor and compare them to other possible candidates. I meet too many students/postdocs who are struggling with an unhelpful advisor but never thought to find out what they were like before they signed up.