Ehh, I feel like there's some challenges there.

In the Toyota Mirai, for example, the tank is located under the passenger's seats, in a nice little 'braced' area. Likely chosen because it's the least likely location for the tank to get damaged in a collision. Also the challenge of either having to 'choose' between a 'swap' tank and a 'fill' tank, or a more complex tank that does both.