The Yike looks unbalanced to me in much the same way as a Segway 'scooter'.
You have your hands on the handlebars and yes you'd think you could get your hands out fornt to break your fall easily, but that fast forward momentum on either the segway or the Yike would I think slam you forward pretty fast if you hit a bump. This is a rather upsetting clip of someone doing a fast faceplant on a segway while riding on an apparently smooth mall floor:
http://www.youtube.com/watch?v=rvWUWtA5jIA
I personally don't think falling forward off a fast moving Yike would be anything like falling off a chair. I imagine it would be more like getting thrown off a chair-catapult.
Riding on a normal bike is substantially different when doing a sudden stop- you have the advantage of having a large wheel in front of you that you can brace back against, and your center of gravity is well behind it. Of course people do get thrown forward off their bikes, but it just looks to me like you wouldn't be able to avoid falling forward at all on a yike if you do anything more drastic than a real gentle stop. You get a lot more leeway when stopping on a normal bicycle.