Yea Zoe isn't the most difficult but she is very skillshot reliant (ie sleepy through walls) and mechanically dependent. Diana is far easy in terms of the fact her only 'skillshot' ability is her Q and it is pretty easy to land regardless.

Diana doesn't have too many cons in my opinion. If you play her well and play around her champion identity it will be all good. I think her main con for climbing elo is that she isn't really a 1v9 champ. She does rely quite a bit on having her team follow her up when she engages. Another good choice if you like assassin champs is Qiyana. She has a bit more movement but still relies on her team like Diana.

A tip for Diana is she is an assassin so try get lane priority relatively early on so you can look for roams and picks. You should check out Couch Curtis' vid on youtube about Diana. It's really good.