Yes, agree that while reviewing schools is one thing, reviewing individual instructors is problematic.
Unless the instructor is freelance, that's like zagat reviewing waitresses instead of restaurants.
Complication... the reviewers (students) in most cases want to get jobs as CFI's when they finish training. This creates a conflict of interest unless the reviewers are anonymous. But aviation is a small world, a typical CFI could probably guess who gave him a bad review... and could then anonymously retaliate when that student becomes a CFI.
Also... students are by definition not experts. There are certainly some CFI characteristics they are qualified to evaluate, but if a student didn't get signed off because he sucked, that's not the CFI's fault.
Apps like zagat (and ratings on amazon, uber, etc) work because there are a large numbers of raters, so any sour grapes reviews get lost in the statistical noise. CFI's typically only have a handful of students, so one bad apple who lashes out at the CFI could ruin that guys rep.