Frill is well suited for beginner or mid-level startups looking to get customer feedback on their products. If a business has a really well-suited product pipeline filled with features and is confused which one to build first, Frill is a really nice platform/product to have. You can get direct customer feedback and you can finally work on things that matter the most. It is less appropriate for someone who doesn't have a big product feature pipeline or roadmap in their mind.
If you have more than 2 developers this thing is basically a requirement within github to manage sprints if you use agile methodology. Why use a different website entirely when you can run your entire board right from github itself? I've tried other solutions but basing everything on github issues makes it so no duplicate work has to take place. If the UX quirks and mobile were fixed this would be 10/10 for me! If you're only one or two devs on a team it may be overkill to use, but if you're going to scale it's better to put the process in early.
Once it's up and running it's easy to use. It needs a little consideration to get set up perfectly for your own needs, but that is the same for any feature-rich software.
Support is good, but quite honestly, I haven't needed any support since 2015. As I remember, I was required to open a ticket and had to wait a few days for resolution. I give it a rating of 8 because of the lag in getting a solid resolution, but it was resolved adequately.
I have used Workfront in the past, which in my experience, is best with a traditional waterfall methodology, similar to Microsoft Project and the other more traditional project management software projects. ZenHub is truly designed around the agile methodology. Other products that claim they are agile oriented seem to just be adjusting and tweaking their traditional product to include a few agile features.