From the linked site:

"Why? Because V8, SpiderMonkey and JavaScriptCore are all too big and complex. MuJS's focus is on small size, correctness and simplicity."

About 'correctness' and 'small size' ... as soon as they'd add unittests as extensive as V8, JSCore, or SpiderMonkey, it'd grow big quite dramatically. :-)

I'd be interested to see how ES5-compliant MuJS really is. My guess is probably different from what they claim.

surely unit tests don't get compiled in?

And JavaScriptCore actually has very solid API, if not simpler.