I just fired 30 rounds of my reloads through a 2nd Model .38 Safety Hammerless 2 days ago (shipped 1888). My reloads are a bit weaker in recoil and muzzle blast than factory loads (which are already pretty anemic), but of course I have no way of measuring actual pressures. I have fired factory loads in both .32 and .38 Lemonsqueezers with no ill effects, but of course you're on your own when firing these old beauties.
This question has come up many times on this forum, with many people saying they shoot smokeless powder loads in breaktop S&W's without a problem. David Chicoine, who's a real expert on these guns, recommends black powder only. I think his concern is not blowing up guns, but stretching of the frame. Most people who shoot these guns don't shoot them all that much, so I don't know how much of an issue that is. As I said, I've never had a problem with several of these guns over the past 10 years. I also don't like buying guns that I can't shoot.