The biggest complaint about Harper is that the quality of grammar checking still needs some work. I've got an exciting new project I'm preparing to discuss later this week (hopefully) in that area. It should be a step-function improvement over our previous methods for creating new rules.
In the meantime, I've collected some recent sightings of Harper in the wild:
Failing to account for this reality can slow down development and dissuade contributors from sticking around.
It didn't work for me, and if you reading this, it probably won't work for you either.
An experiment on how to live in a seemingly hopeless world.