Personally I don’t see the necessity of this unless you are on some remote expedition or in some wilderness area etc. MUCH cheaper to have a good supply of SD cards on you…
I wouldn’t recommend this product for simply storing more data. It would be most useful for someone who needs to review 4K footage right away out in the field, show the footage to clients in the field, or retrieve and edit footage while out in the field. A fist full of SD cards is the way to go if you don’t plan to look at the footage until a later time.
And a sunshade then! 
Best bet, sd cards and a Crystal Sky…
I agree wholeheartedly and often my clients will want to see the raw footage… I like the sd slots on the CS… and high res image the CS produces…