Kobo, Kobo...
With EPUB, there is an obvious, long lasting problem with side-loaded fonts (italic to bold) it's also very tricky to use embedded fonts... Also I noticed -many times - a big bug regarding the display of long paragraphs..
But, I just learnt today that I can transform my EPUB as a kepub.epub. I tried it and for the first book:
The fonts problems (the main ones as far as I can see) seem to be solved. But... there are superscript defects, problems with centered and right-aligned text. Also I don't like the title display up and down... This after just five minutes...
For both EPUB and KEPUB, I deeply dislike the slider to go from one page to another. It's vastly innaccurate. I remember on the old PRS-505, you just typed the page number. Precise, simple. Un-Kobo probably.
These are not glitches, but glaring reading defects. I lost confidence.