Stem shifter have been known to cause serious injury to sensitive areas in collisions - even worse were the top tube mounted shifters of early 5 and ten speeds and "stick shifts" on muscle bikes - which is why they were banned from the U.S. marketplace.

It's not a matter of going over the handlebars, but if you hit something and slide forward off the seat, you could be in for a pretty nasty injury.

But since you can pretty easily change them out for down tube shifters, it shouldn't really deter you from a bike that you otherwise are interested in.