I guess the danger involves the need to flush to begin with.
If an engine has had appropriate change intervals with the appropriate oil through its life, it will not sludge.
It may be that some engines really need frequent changes with expensive oil, but these are pretty well known, and farily rare.
Regardless, with proper maintenance, sludge will not be an issue.
Without proper maintenance, a flush is probably asking for trouble, and had there been proper maintenance, there would be no need for the flush