That's not so hard to guess: triiforce doesn't work on revisions 18 and 19. More specifically: it needs revision 17 or lower in slot 249.
On the other hand, rev19 is better in some instances (base IOS37 to allows to play DLC from music games). As such, you want to have both installed. And considering you can't choose which slot to use in triiforce but you can tell USB loaders to use different slots for different games, the result is that installing rev19 in a slot you otherwise don't use isn't such a bad idea.