I have been using Stylo for some time now. I have faced exactly zero problems. I think it's rock solid.
I don't think it's the default CSS style system though. You can enable it using layout.css.servo.enabled to true in about:config
You can already enable this flag and Firefox stable, but I don't think it takes effect yet.
It only works in Firefox Nightly. The flag exists in Beta and Stable, but since Stylo isn't compiled with these versions the flag doesn't actually do anything.
Are you sure? I thought I saw a FF dev mention right here that the flag is available in stable just a couple days ago.
From what I can tell, Beta 56 has it (but you have to enable it), Nightly 57 has it (https://bugzilla.mozilla.org/show_bug.cgi?id=1330412 is tracking when it'll be enabled by default), and Stable 55 does not have it at all (although the pref might be there).