These days, I mostly carry my larger revolvers, in one of the "Tanker" type shoulder holsters. More for practicality, and not to concerned about concealing them.
Pacific Canvas and Leather....
El Paso Saddlery....
Back in the 70s, for a short while, I carried a S&W 19, 28, and 29 in Bianchi X-15s. Didnt take to long to figure out, there were better ways.
Ive also used a Ken Null "SMZ" with my J frames, and Walther PPK/SIG P230s.
The Null, and the Galco Executive's, were really the only concealment type shoulder holsters Ive ever really liked. They work well and conceal much easier than most of the other types. Then again, they are small, minimalist type holsters, using fairly small guns.
As for safety issues with shoulder holsters, they really are no different, or less safe, than any other method of carry. Unless you consider your trying to conceal one a safety issue. In that respect, they can be an issue, as most dont conceal as well as other methods.