It can expose leaks that were already there, and this is worth being straight about. On a very old system, corrosion thins the metal from the inside, and the sludge sitting in the radiator can be the only thing plugging a pinhole. Clear the sludge and the pinhole starts weeping. The flush did not cause the corrosion — but it is the moment the damage shows up.
What a power flush will not do is damage sound pipework. Modern flushing pumps are high-flow but low-pressure, running well below the pressures the system already sees in normal use. The risk sits entirely with metal that was already failing.
So before we flush anything we look at the age of the system, the condition of the radiators, the state of the joints, and whether there is any history of pressure loss. If we think an old system is likely to start weeping, we will say so before we start, and we will talk through whether a flush, some radiator replacements, or a wider overhaul is the sensible spend. On a system that fragile, spending £500 on a flush is often the wrong call and we would rather tell you that.