The one I saw was built by Linebaugh. I believe he said it was a concern to try to fit all the rims in on a 6 shot, but apparently its been done. I dont recall if he had other concerns, this was maybe 25 years ago. The primary downside, youd have as much in a cute little 38 or whatever single six size gun as a 500, it takes about the same amount of work. Id be tickled with a genuine Colt SAA in 357 as a fun shooter and carry gun. I saw one at a small cowboy shoot years ago in Az, the lady that had it said someone slicked it up. As i was admiring it, she asked if i wanted to shoot it, and handed me a 100 rd box of shells and said to use them up, there were plenty more at home. Youd still have far less money in it than a custom Ruger 38/9/38super or whatever. For some reason, reading that "Colt's Pt FA Mfg Co Hartford Ct USA" on top of the barrel still tickles me.
Having said all that, the old early 1950s 3 screw Rugers when they made the 357 on the smallest frame they did at the time was a very sweet handling gun, not as bulky and heavy for caliber like the full size blackhawks can be. I dont know if the flat top 357s of recent vintage have that lighter feel.