Sad story, I had used Karma from the start, and still using it. I believe that Karma was one of the tool which made web unit testing mainstream, easy, robust and joyful. Because I had contributed to the Karma and had setted up Karma in a lot of different environments. I can say that Karma is still having the best architecture even 10 year later. Thanks to Vojta Jína and Co. Every time when I use Jest it felt bad, it is maybe easy to start in first place but has disadvantages:

1. Jest is terrible slow

2. Jest running on Node.js, and it works pretty bad even for web unit tests

3. Karma had unique feature that you can connect any browser and run test on them. I hope Web test runner has such feature