Stem shifters, as I was told by an old timer who has worked in bicycle shops for close to half a century, tend to raise the rider's center of gravity, to implement shifts. Down tube shifters, when used, help to lower one's center of gravity. A lower center of gravity will increase stability - dramatically.

Ugly - yes, but that is only an opinion, not a fact. Heavy - yes. Dangerous, compared to any other shifter style - yes. Looked down upon, thanks to the common use on low end bicycles - yes. Easier to use - no. Hands must leave the handlebar, another dangerous activity - yes. Are they the least efficient of derailleur shifters - yes. Do the mounts tend to fail - yes.

Other than that, nothing wrong with them but I absolutely assure you that I would never mount a set on a bicycle, even if I were selling it to someone else and that, too, is a fact.