Yes, if you install a third party keyboard one of the new features in IOS8 and give it full access. You also agree to send everything you write to a third party. Not only do you agree that the developers of the keyword can read anything you write, they are also allowed to send the information to their own servers for processing and once what you have written is there. You have no chance of knowing what they will do with your written passwords, credit card numbers and all other personal information. And even if they don’t do anything bad with your information, you always have the risk of them being hacked.
So if you care about not getting your passwords and credit card numbers spread across the globe I suggest not allowing any third party keyboards to have full access.
Apple say that they will switch to the native keyboard when you are about to write sensitive information but can they garantee that?