Namely, your defense against legally compelled decryption is significantly weakened via use of a fingerprint sensor, since the phone's unlock mechanism is the same authentication system that the phone's encryption mechanism uses (it would be really nice to have some sort of separation there).
The last drawback, I'll simply quote verbatim from the article:
>What would happen if some government “asked” phone manufacturers to create a back door to store or send it fingerprint information, and to lie to the public with denials of the existence of such a program. Considering the news of the past few years such a scenario seems far from impossible.
On that note, I wonder how compatible smartphone fingerprint data is with data you'd find in a fingerprint database. If not immediately compatible, would it still be possible to convert it to a standardized format?
[0] http://www.forbes.com/sites/josephsteinberg/2015/03/05/wh-yo...