In order to enable the extension keyboards functionality you must "Allow Full Access." This is where the problem lies. As seen in the warning below, turning this feature on allows every keystroke to be sent back to the developer.
Just think about that for a second... What do you type into your device? Absolutely everything.
I hope that this warning is just Apple's due-diligence, however it seems that as though this is the next data mining jackpot.
Please take your security seriously, and take a moment to think before enabling the "Full Access" feature.
Here is Apple's full disclaimer about the Third-Party keyboards which can be found on you iOS device under Settings>General>Keyboards>About Third-Party Keyboards & Privacy...:
"Third Party Keyboards provide an alternative way to input keyboard data. These keyboards can access all of the data you type, including bank account and credit card numbers, street addresses, as well as other personal and sensitive information. These keyboards may also access nearby text or data, which is useful for improving autocorrect functionality.
If you enable Full Access, developers are permitted to access, collect and transmit the data you type. In addition, if the third party application containing the keyboard has your permission to access location, photos, or other personal data, the keyboard can also collect and transmit that information to the keyboard developer’s servers. If you disable Full Access for a third party keyboard, and then re-enable it, the keyboard’s developer may be able to access, collect and transmit what was typed while the network access was disabled.
If you do not enable Full Access, developers are not permitted to collect and transmit the data you type. Any unauthorized collection or transmission of this data without your permission would be a violation of their developer agreement. Furthermore, there are also technical limitations in effect to prevent unauthorized access. Any attempt to defeat these limitations would also be a violation of their developer agreement.
You may choose to disable a third party keyboard at any time. To do so, open Settings, tap Keyboards, and remove the keyboard from the Keyboard list.