Kindle devices now support the open epub format which is a improvement at least. Kobo devices come chained to Rakuten so it's not a big enough difference for me to consider switching... for now.
You still can't load or read epubs directly on Kindles.
To the average reader, does this have an negative impact?
This doesn’t appear to be true. I have several non Amazon epubs that I sent using the web interface, all with covers.