There were a few Spanish makers who could be called "second tier", but most of those revolvers were of a lot lower quality and were made specifically to sell in the U.S. where proof testing was not required and there was no government quality inspection. Most were made of pot metal. That term, in spite of modern claims otherwise, does not refer to metal melted in a pot but to the metal that cook pots were made from, cheap cast iron. Many makers did case harden soft parts, and the guns will often stand up fairly well as long as the ammunition is low pressure.
Like other material, iron will stand up pretty well to pressure as long as not too much is asked of it (sort of like people, maybe). But I have seen some of those guns that blew up, a couple with unknown loads, one with definitely a standard load and one with a blank.
Most of the makers went out of business during the Spanish Civil War, and when it ended, Franco wanted to restore Spain's arms making industry and reputation; he picked three of the best makers and ordered the rest to cease operations,
S&W did take an interesting step against the Spanish makers and the importers. They trade marked the color case hardening of the trigger and hammer. Note, I said "trademark", not patent. Patents expire; trademarks do not as long as they are defended.
By trademarking the colored parts, S&W put the importers of Spanish revolvers in a bind. If the makers didn't color the parts, the guns wouldn't look like S&W's. If they did, the importers could be sued for trademark infringement and put out of business.
And that is why even today S&W colors their MIM triggers and hammers, even though they are hard all the way through and don't need case hardening.
Jim