The "problem," with regard to KOReader as well as similar third-party software, is a lack of reverse-engineered support for E Ink updates on Onyx devices. Whether that's an actual problem or just slightly suboptimal will depend on the individual device. (One day it might come, if someone with one of these devices puts in some of the necessary effort.
)
Additionally, Android devices are typically quite slow in spite of their beefy hardware.
So the Kobo Clara HD is likely already preferable just by itself, arguably also for Kobo's software and not just for KOReader, coolreader or Plato, and when you take into account that the Onyx Poke 2 is 190 while the Kobo Clara HD is 110 it's very hard to see much in favor of it. But that doesn't mean you shouldn't be perfectly happy with it.