The issue with lead bullets in guns with polygonal rifling, such as Glocks for example, is that the bullets tend to skid against the rifling, both at the beginning of the rifling (the leade) and the end (muzzle). This can cause lead buildup in these areas, which if not removed, can cause increased pressures. Too much buildup, as you can imagine, could be a real problem (boom). It is NOT a problem, though, if any leading which might occur is cleaned out of the barrel regularly.
It is by no means certain that leading will occur in such barrels, though. As mentioned by others, bullet fit, alloy used, powder speed and loads not beyond what the bullet alloy can handle will minimize the issue, if it occurs at all. Lead deposition can occur in ANY barrel, with ANY type of rifling, if the conditions are suitable for it.....it is simply that polygonal rifling tends to be somewhat more subject to this phenomenon
Regardless, diligent maintenance is the key.