Hi Eorar,
Sadly, this comes from what I call "people thinking they know everything" syndrome. People tend to forget that the world has been so shattered, so sequestered, that races are not common knowledge unless you've basically grown up with them. And since most of these people haven't grown up with such rarities as Dhani, they are very much overstepping their bounds on 'knowing' them until they've earned it in game. Sure, they may have a reputation, but that depends on where you are and where they're from, what sort of trouble they cause, and, quite frankly, unless you're from Taloba or Ahnatep, it's unlikely to be preceding from there.
In their human form, Dhani are essentially indistinguishable from normal humans until they open their mouth and speak. Then their "S"es tend to give them away, but again, PEOPLE SHOULDN'T KNOW ABOUT THIS, because there is such a condition as a lisp, after all.
Dhani are not common.
Myrians and Eypharians, as you said, would know about them - Myrians know about the three subraces because their ongoing, subtle war dictates this, though they are most familiar with the constrictors. They know because the Constrictors have thrown 'ambassadors' of the other subraces to the Myrians to save their own skins, as it were, and that knowledge has been recorded by the Myrians. Do they know all the secrets? No. But they are aware that there are two other types. Stab first, question later.
The Eypharians would be likewise mostly aware of the Vipers, but some who interact closely with the Vipers and their Nest may occasionally note the distinction between the Dhani, but how much they know of each subtype is probably not much, since there is not a lot of traveling between the nests. Times are too hard, numbers are too few, and resources are too scarce.
Dhani, however, do know about the other subraces and likely consider them inferior to their own.
Does this help?