iOS as a general platform tends to blast API's more quickly than most platforms--take a look at all the top-line games that needed special versions for iOS 8.
In addition, Apple has it's own "private" API's that app developers aren't allowed to use which makes development more complicated than it has to be.
Finally, iPad's have way more screen size than iPhones. Little things that you might get away with on a 5 inch screen suddenly look really bad when blow up to 10 inches.
Citation needed? What do you find buggier about iPads than other iOS devices, and what makes them harder to develop for?