I can read German but writing is a bit more complicated for me.
The main problems for Kobo devices are:
• a huge header (kepub) and footer (kepub and epub) on every page. With to much space and postioned too far from the edge of the screen. Leaving not much room to display text and images for its main function:reading
The actual screen size for Kobo readers is for
kepub: 79.785%. (header and footer)
epub: 90.527% (footer only)
Ereaders from different brands score often better than 90.537% when displaying both a header and a footer because these footers are not bigger than necessary and are positioned closer to the edge of the screen. Other brands offer also a full screen reading mode which Kobo does not.
• annotations/highlights/notes. These are not stored in the correct order or order changes freqently. If you add or delete a note/highlight the order is shuffled.
• embedded fonts (supplied by Kobo and part of the firmware): sub fonts not displayed correctly. Italics or bold or bolditalic are displayed often as regular. Kepub files and epub files produce different outcomes.
•pdf. When a pdf contains pages with different page dimensions larger pages are not displayed in full if the first initial page is smaller. The scroll function doesn\\\\\\\\\\\\\\\'t work. And pdf reflow fails, which is key for reading pdf on small screens. 6.8\\\\\\\\\\\\\\\" is still a small screen.
•kepub display issues, like empty line after every p-tag. Centered text is displayed as left aligned.
•metadata of books is very limited. Only description, file format and language are displayed (epub). When it is an kepub the ISBN and Publisher are also displayed.
Kobo isn\\\\\\\\\\\\\\\'t very good at software/firmware.
• If the try to fix a bug they will almost certainly break something else. Or move the bug around. They tried to fix the embedded font issues. If they fixed i.e: font 1, font 4 may contain the same bug (which was fine in the previous firmware release). In firmware release 3.5.0 almost all fonts were (again) broken.
• reintroducing bugs. The empty line after a p-tag was reintroduced in firmware 3.8.0 when displaying the description metadata.