Known problems 

Input of a rejected new key is lost
When a new key breaks a rule, FormEngine opens an empty form again. This is how FormEngine handles every record that DataHandler refuses to insert. An edit of an existing key that breaks a rule is dropped as a whole.
Error messages carry a generic prefix
DataHandler wraps every save error in "This action could not be completed; please contact your administrator". The reason follows after "Technical details".
Keys created without a backend session can't be shown
A key record created by DataHandler outside the backend, for example in a console command or an import, gets a hash but its secret is never shown. Use apikey:create on the command line.
Backend users can't manage their own keys
Key records are admin only, and the module lives in the admin-only Integrations module.
No rate limiting
Failed attempts are logged but not throttled. Limit requests in your web server or proxy for now (#8).
Bearer tokens behind Apache
See Apache and the Authorization header.