A few of the JS polyfills for JSONWire behaviour have been fixed to do nothing in edge cases, and the POD advises on how to avoid said cases in the future.
is_enabled was broken in 1.21, it is now fixed.
Passing extra_capabilities to firefox and chrome was previously broken. This has now been remedied, but in a non-optimal way due to the difficulty in figuring out ahead of time whether a server used supports WC3 webdriver or not. Hopefully in the future we won't need such bogosity.
See the POD's COMPATIBILITY section for more information.