Invisible - if you're low enough, flipping isn't an issue, but getting that low requires more than a 40 inch wheelbase or you won't fit between the wheels. What you have to consider is the angle between the center of gravity and the front and rear axles. Get the line too steep and it doesn't take much acceleration/deceleration to lift the other wheel.

Cruzbikes have their plusses and minuses. They can be inexpensive, but they are very different to ride. I think the principals at Cruzbike have spent time here, and if they show up they can provide more info. Lots of owners like them. I'd say the learning curve is very steep due to the moving bottom bracket. You have to move the pedals and boom to balance. At first, even coasting is tricky, much less climbing anything steep. I tried one, although not for long enough to learn the tricks in riding it. The owner can keep it. Apparently, once you learn, it's not an issue though.