The other big advantage is battery life. I have an old Kobo Aura I got second hand on eBay. I keep the WiFi turned off, have installed KOReader and load books over USB (with the held of Calibre). With semi-regular usage the battery lasts weeks if not months. Way longer than any phone or tablet I ever had. Granted of course those devices do a lot more, but it's nice to at least very rarely have to worry about whether you have enough charge to read a book.

Like you I'm not sure of the advantages of eink for more general computing. I wonder what the (actual) battery life is like on the ReMarkable.