I have a Kobo reader--and download/purchase from various places without any problems. That being said: Kobo does not allow two ADE accounts to exist on the same reader. So, if I download with an ADE account
sss@lmail.com and I use another account at another store (sss@yoboy.com), only books loaded onto the Kobo with the ADE scheme used last will be available on my reader--content will be locked for the other one.
It's not a problem to always use the same ADE account--but there are some bookstores that try to get you to use an account they set up from you. Sony books does that and it took me a bit to get it all sorted out.