This is a valid concern, and we'll do our best not to screw up. :-)
It's not as hard because there is no parallelism (at least now) and any mutations of the internal data structures happen in two or three tightly controlled places. But yea, debugging these is harder during development (for us) and we'll need to make sure our testing story is rock solid before shipping the asynchronous mode by default.
One thing that really helps here is we test all updates on Facebook which uses React in many places and has a giant user base. So regressions that lead to a drop in metrics (e.g. a crash or a race condition) automatically get reported to us, and this adds a second line of defense after unit and integration tests.
It's not as hard because there is no parallelism (at least now) and any mutations of the internal data structures happen in two or three tightly controlled places. But yea, debugging these is harder during development (for us) and we'll need to make sure our testing story is rock solid before shipping the asynchronous mode by default.
One thing that really helps here is we test all updates on Facebook which uses React in many places and has a giant user base. So regressions that lead to a drop in metrics (e.g. a crash or a race condition) automatically get reported to us, and this adds a second line of defense after unit and integration tests.