Regardless of what a company claims about ownership, it is possible for you to lose access to any book that is protected by DRM (at least with Kindle and Kobo, which authorize against a server). This is troublesome if the vendor goes out of business, drops out of the ebook market, drops support for a particular version or type of DRM, or simply changes company policies. Given the current state of copyright law in some jurisdictions, such as Canada and the United States, it is also illegal for you to remove the DRM in order to protect your investment.

That being said, both Kobo and Kindle support ebooks that are not protected by DRM. The readers will certainly give you access to such ebooks from third parties, and I believe that both vendors provide books without DRM at the author's request (i.e. look before you buy).