Depends on what you mean by "hard". Although it's time consuming to learn different platforms, there are not a whole lot of gotchas you have to discover on your own. It's all been done before and well documented.
PhoneGap development is hard inasmuch as there are tons of gotchas you don't know about and haven't been clearly and extensively documented.
PhoneGap development is hard inasmuch as there are tons of gotchas you don't know about and haven't been clearly and extensively documented.