Dragons take the forms of mortals for social and/or strategic reasons. When they do, they take the form of "regular" mortals i.e. without draconic features, because that would ruin the point of taking the form to begin with. Socially, the dragon may take the form to be more presentable to their mortal allies in public and not draw attention to themselves, which leads to the strategic reason; they wouldn't want to give away the fact they are a dragon to potential adversaries. Of course, the Dragon Aspects are exceptions to this rule. Their mortal avatars are somewhat "enhanced" though note that these forms still don't bear particularly draconic features, other than Alexstrasza and Ysera having horns in their forms.
The Drac'thyr use this illusion magic only to take the forms of mortals with draconic features, however. This makes no sense to me. The Drac'thyr are meant to be great soldiers for the Dragonflights, so their use of said illusion magic should be even more utilitarian than the dragons themselves, who as stated before, often take the forms of mortals merely to satisfy social requirements. Drac'thyr would want their visage forms to blend in with the mortal populous for strategic reasons. For this reason they should be able to take the form of most if not all mortal races, with no added draconic features giving away their true nature.
Then again, Drac'thyr can't be rogues so they aren't capable of subtlety anyways. Ba dum tss
Edit:
Source -- Refer to section titled "Shapeshifting"
Furthermore, by bringing up points like this we have a better chance of having an influence. Look at all the YouTubers that covered the Dragonflight leaks lol. And they're already talking about the popular opinions that the Drac'thyr models need some work.