For the above reasons, this practice has been documented to raise pressures, occasionally significantly and dangerously. The jacketed bullets don't have sharp leading edges, so they burnish the lead into the bore and smooth it over rather than scrape it out. It acts sort of like a very thick grease, but one that doesn't evaporate as you shoot.
Back when I first started shooting centerfire revolvers, I was told to shoot jacketed bullets after lead ones to clean barrels, and I'd look down the bore afterward and it looked smooth and shiny and I thought I had cleaned it. Then one day I ran a clean bore brush through one these "clean" revolver bores a couple of times. I can't recall why. Looking into the bore with a bore light, that shiny smooth looking surface had all turned matte and dark. It was the lead surface the brush had scratched and raised a little.
Most guns are made stronger than is strictly required, and that saves a lot of people from getting into too much trouble with these kinds of ideas. But a gun that fouls badly enough can accumulate enough lead that the barrel starts springing out as the jacketed bullet passes through it and over the lead accumulation. It can make bad over-pressure trying to flow a thick layer of lead down the tube. Besides, why put even a gun that will tolerate it under that extra stress? You just need the Chore Boy, a Lewis Lead Remover, or some NO-LEAD. It'll come out.