Kindle, Kobo etc
The cheap kindle is only 6" and 167 dpi.
Better Kindles and Kobos are 270 to 300 dpi.
Font support is limited. Only serif, sans and mono can be reliably used on all platforms. Stupidly the early Kindles couldn't (and still can't) do non-Latin fonts
Yes forget about images.
Ironically the internal content of ePub and mobi/AZW is basically HTML and "responsive". But eBook GUIs and the management on device are garbage, like some sort of DOS program. Internally all eBook readers use either Android or Linux, so the 10 year ago lack of support for Japanese, Chinese, Korean, Cyrillic, Hebrew and Arabic was inexcusable.
Basically Amazon ported the Mobi reader and since has hardly more than updated DRM and spyware. It's sad how crap the Kobo and Amazon software is.
Anyone with an eReader, or reading eBooks on a phone or Tablet App needs Calibre (free). Lets me read Amazon Kindle books on my Kobo, my old Kindle DXG is too heavy and awkward for annotation.
The Kobo Aura H2O is better than Kindle Touch for annotation via Calibre.
It's a pity Amazon never did audio support properly and then ditched 3.5mm jack.
It's a pity Amazon is ripping off readers, authors & publishers with the subscription service / Prime / KDP Select.
It's a pity Amazon now sells eBooks that don't work on ANY eReader, only on an App.
It's a pity Amazon now have a format that if you download direct to eReader (3G or WiFi) you can only ever use their so called Cloud to transfer if ereader lost/broken and can't personally backup. Always choose download via PC, transfer via USB and keep WiFi/3G off on any eReader from any maker. They want to spy on your reading habits.
A broken dream, due to greed and carelessness.