I've heard that, too, but I have no first hand knowlege. Anyway, the conversions, of course, remove some data that takes up a lot of space, but whose lack thereof minimally degrades the music. Which is the long winded way of saying that they hope that you really can't hear much difference, if any. However, they each, in their own way, use different criteria on which data to remove and if you sequentially perform both conversions it does matter. In so doing, you've lost two different sets of data and the combined deletions do noticeably alter the music.
Certainly, the effects vary according to the nature of the music. Hip-hop music is only degraded in a minimal fashion while real good music like that of Merle Haggard or Waylon Jennings is totally destroyed.
Who is having trouble finding his Hank Williams (Sr.) 78s.