ESR is a little grandiose. Note he doesn't give much detail, just say that it will bite you and you should listen to his assertions because he has done it more than anyone in the world.

You're not exactly wrong, but the test suite for Reposurgeon has a number of test cases derived from malformations caused by other tools, or made worse by other tools. The real problem here is that they were gathered over a decade, and nobody (myself included) really documented them very well. They all made sense when we committed them…

Also, most conversion tools will handle the most common cases OK, and only generate garbage in oddball situations. They don't print out a warning when the oddball situations happen, so unless you go back and look at every commit you'll never notice the damage. Most of those situations only happen because of accidents, like creating a Subversion branch the wrong way, then deleting it and making it the correct way. Or adding files to a CVS repository by copying ,v files from a different CVS repository.