I remember back when those were popular (or at least common). Some manufacturers would put the stem shifters on their cheapest bikes and downtube shifters on their better bikes. The problem with stem shifters is that they are hard to reach except when you hold the handlebars from the tops, which gives you an unstable, unaerodynamic, and and inefficient riding position. If you do ride like that and hit a pothole (or car or dog or curb), it is pretty easy to get thrown over the handlebars, then the shifters sticking upwards will impale your chest. Some stem shifters have rubber ends to reduce the impaling problem a little, though flying over the handlebars is still not fun.