The big downside to C++ on iOS is, well, XCode support for C++ stinks. I would go far as to say XCode stinks, but some people seem to like it for reasons I can't quite fathom.
What? XCode has very good C++ "support", using LLVM/Clang for compilation and static analysis. They've a quite nice integration going there, really, so where are you getting your facts? And that last part -- I like XCode, in fact, I prefer it highly over Visual Studio and I'm a professional (if that lends some kind of magical power to my opinion). If you can't fathom that, I'd say you're far from reality and just limiting yourself out of some illogical reason.
/Going way OT
... alright, riddle me this then. Describe the process of creating a C++ game in XCode... what options do you pick? This perfectly illustrates the importance of C++ in XCode.