Hi Dinth: KOReader works very well on kobos, bq cervantes and *some* android devices because these are currently maintained platforms.
Kindle is maintained too, but amazon adds restrictions which are difficult or impossible to fix.
Pocketbook is not maintained as nobody contributing to KOReader has one of these devices.
Just to figure out different issues on different platforms:
open issues for kobos
open issues for kindles
open issues for pocketbooks
As you can see the pocketbook platform is a mess in terms of support and this won't change unless somebody starts to contribute for that platform.
So, if you're happy with how koreader behaves on your kindle (and can live without other features that PB has) stay with it or try a kobo.
If you can live without koreader (I guess you can
) and find your pocketbook software nicer then it's up to you, but don't hold your breath waiting for future platform support because it might never happen.