As Ellipsis pointed out, this was just one very simple example. There are hundreds if not thousands of differences in the overall toolset between the different specs that will make designing a manageable number of experiences basically impossible. Mages have like 9 defensives while a balance druid basically has one (unless sitting in bear form is a viable option while meeting damage checks). Vengeance demon hunter has like a half dozen stops while brewmaster monk has 2. Even racials come into play as stone form from dwarf is a significant defensive against most status effects while arcane torrent from blood elf is niche at best.
Like I said, the only way to even remotely make difficulty parity between solo and group content would be for the solo content to truly test effective use of all tools in that player’s toolkit. But there is so much variety in the available toolkit between different specs even of the same role that there would likely have to be an experience created for each spec.