which, if you rely on it, defeats at least one "end" in "end-to-end" if you don't know what device your peer is using.
So much for end-to-end encryption in Whatsapp...
That's exactly what I was thinking. If WhatsApp was an Android-only app, it might be able to claim end-to-end security. Even then a quirk in a certain version of Android on one end, or even a hardware quirk with a certain model phone, could impact security.
Of course if you want the most secure communication possible with someone, you won't use a smartphone in the first place. There isn't a cellphone on the planet that's 100% secure from eavesdropping.
I've never used Whatsapp, but maybe they inform you whether or not the other end supports and is using encryption? It would certainly be easy for them to do something like that, hopefully they do.
http://www.heise.de/ct/artikel/Keeping-Tabs-on-WhatsApp-s-En...
tl;dr end-to-end encryption in WhatsApp is not really useful (yet)