I expect that those who make their own in soft alloys or pure wheel weights have gone to aftermarket barrels.
Might be the case in general, but I'm shooting pure WW in a stock Glock barrel, and I have no reason to clean my bore or chamber, at all. Indefinitely. Thousands of rounds. Actually, I avoid shooting as much with my LW barrel, because the button cut rifling still picks up a tiny bit of leading in the rifling near the chamber. Stock Glock barrel fouls none, at all. And I load well hotter than most, from what I have gleaned.
The harder alloys, generally purchased bullets, act more like jacketed and are probably less likely to cause a leading problem.
I agree that the harder the bullet, the less likely it is to cause a SEVERE leading and accuracy problem. But IMO, they are equally likely to cause a problem, however mitigated it might be. Before I started to expand my 9mm with a 38SW plug (I had the same issue with 9mm as Arkansas Paul in post #36), I was getting much BETTER accuracy and much LESS fouling with harder MBC bullets. But in this case, I was still getting a huge falloff in accuracy past 25 yards and enough leading where I needed to either clean within a hundred or so rounds, or to mess with mixing jacketed bullets in each mag to keep the bore clean.
Once you have determined your gun can shoot cast bullets with no fouling, and you can load them without swaging, and you have figured out at what size, then the extra hardness pretty much does nothing for you. That's if you can reach this point.
This is significant for the reason of cost. While pure lead generally costs much more than "dirty" lead, when buying dirty or alloyed lead, the cost goes up as you increase the hardness, because the alloying elements that increase hardness cost more than the lead. Also in some rare cases, a softer bullet performs better due to its ability to overcome minor constrictions/irregularities of a non-ideal cylinder throat or bore or a minor undersizing. Or, perhaps, the oversized chamber/throat of a stock Glock 9mm/40 bore. (The 45ACP Glock chamber/throat is much less oversized, in my examples!)
Just because MBC makes x bullets with y hardness doesn't mean that's what works best. It's just the best hardness for their particular application which includes selling bullets to thousands of customers and being exposed to some amount of liability and headaches in case said bullets were to cause problems to some of those users.
If the bullet is not big enough to seal the bore, my soft bullets will lead a Glock bore, end-to-end, with one shot, and you would be lucky to hit a barn door at 100 yards. Shoot a mag, and you will be scraping solid lead out with a sharp stick, and you still probably wouldn't have hit that barn door, yet, and you better hope your backstop is 50 feet high. I've been there and done that. So, no. I wouldn't sell or give these to other people and tell them to go ahead and enjoy with no caveats. And I imagine professional sellers of bullets don't have enough time in the day to answer the unhappy phone calls, either. But when loaded correctly they work perfectly in my guns, including and especially stock Glocks. Glocks with stock barrels are my favorite shooters, and one of the reasons is they shoot cast bullets so cleanly and accurately. If you have the time and inclination and methodology (or just luck) to safely get to that point, you will see for yourself. You just might find yourself with zero fouling and soda can plinking accuracy at 100 yards.