I think Kobos use a different rendering engine for KEPUBs than they do for regular EPUB
They use Adobe's RMSDK renderer for epub2 files and a webkit based renderer for .kepub.epub epub3 files.

RMSDK is still needed for Adobe's ADEPT DRM.
I know nothing of hinting, kerning, etc - but is one of the ChareInk versions better for webkit-based rendering?
Try the two hinted versions and see how that goes. Do you have the patch to allow advanced font control on side loaded fonts? If you do, the NoWeight version should work.
I don't recall whether I'm using NoWeight or not. I did patch for Advanced Font Control, as well as to standardize font sizes between ePub and kepub and turn on constant sharpness.

I think I need to check a few more ebooks. I tried a new one last night and ChareInk looked better, so there could be an oddity with the ebook I was looking at - either a conversion issue or something else.

Thanks for the nice work - I loved this font even before I discovered your version.