App is fun but Do your own research! You can find this on macrumors.com among other places... I chose no and deleted the app.
Ultimately, what it comes down to is trust. Granting a keyboard full access certainly does mean that any typing data can be transmitted back the developers servers for a variety of uses. Most high-profile firms have a vested interest in maintaining user trust and being forthcoming about how that data is used.
So while there are potential risks to granting a keyboard full access and some security researchers thus recommend not using any keyboard requiring full access, users weighing the usefulness of a given keyboard should at a minimum be informed enough to consider the level of trust they have in the developers behind those keyboards before deciding whether or not to grant full access.
CDOPHX about EmojiFace Classic