Karma was built to run all of your browsers tests in as many different browsers as possible.
Wallaby.js focuses on seamless integration with many code editors and on delivering instant feedback from your (both browser and node.js/io.js) tests.
Even if it does run in a real browser, cross-browser issues are common enough in my work that my TDD loop always includes multiple browsers. I wouldn't be willing to give that up for what Wallaby offers.
Part of the reason I'm interested in something like Wallaby is that running in multiple browsers is slow--but if I could run just the right subset, it would be fast enough for my needs. (And Karma still runs about 200 tests / second on my mid-2002 MacBook Pro, across 10 browsers, including IE VMs, so it's not like Karma is that slow.)
Karma is not slow, but some of our tests may be. Wallaby just allows not to run them if we are not editing anything that affects them.
Cross browser issues are always a pain, I know what you mean. However, I have realised that I can write the bulk of my code without worrying too much about them.
When I'm writing the bulk of logic, especially with TDD, I found that it's more efficient to run tests as fast as possible in the fastest engine and care about running across all browsers much less frequently. Especially if you run tests as you type.
Later on, closer to the end of the iteration, when preparing to commit my changes, I surely run all of the tests in all browsers/smartphones/tablets that I support. Typically it just works, especially when your codebase is mature, you already know the majority of cross-browser issue sources and have something in place to avoid them. Even if it doesn't work, the issues are normally easy to solve. Overall, less context switching, more focussed work.
Anyway, it's just my experience (you can read more about the motivation behind wallaby.js in my blog post - http://dm.gl/2015/01/30/wallaby/) and we are planning to support more browser runners, or even just karma as a runner for wallaby, in future. Please feel free to vote (https://github.com/wallabyjs/public/issues/29) or create new feature requests.
Fast tests (< 1-5s) have a lot of value to me (although "test when I press save" is as good or even better than "test as I type," as far as I'm concerned), so Wallaby is something I'm interested in. I'm more likely to do manual test sub-setting than give up my cross-browser tests, though.
One potential customer's perspective; you can take it or leave it. :-) I'm not necessarily a good example of the mainstream. I've voted on the issue.
http://i0.kym-cdn.com/photos/images/newsfeed/000/538/731/0fc...