Having had both the IQ and Credo backs in for testing at our studio, we have found USB3 to be "wanting" on several fronts, especially compared to FW800.
1. USB3 does not provide enough power to charge the backs
-real world result = dead batteries in the middle of a long day of shooting
2. USB3 does not allow cable length longer than 9ft
-real world result = cables run across floor and can be a tripping hazard or we now must purchase specialized new cables and active extenders to boost the weak signal
3. USB3 is just as fast as FW800,but not faster, and can not handle sustained thruput the way FW800 can with aplomb.
Not sure whats so great about it other than the first word in its TLA which is that it is "universal". They should have gone with Thunderbolt but whatever...It works and will have to get use to its quirks and live with it.
The Hasselblad looks like a fine system and at that price, amazing! It was just an overall system and support decision for us but if we were a Hassy studio I wouldn't have given it a second thought!
I would tend to agree with your point about not charging. I have used a IQ260 tethered via USB3, to a Surface Pro 2, all field use. The tethered connection does seem to "extend" the normal battery life, which on a CCD back is much less than the CMOS 50MP backs. I base this on the fact that in the field with 2 3400 mllamp Phase batteries, I can shoot all day, 8 to 10 hours, and not need any other batteries, which before would have been needed. Around 5 more. I tend to shoot tethered for setup, then move to the card for the remaining shots, to save on SPro2 system battery life.
I also believe that in warmer conditions, say outside temps of 85 degrees F or higher, and over 75% humidity, the charging option may cause the back to heat up excessively and that REALLY created a lot of noise. Back in September, I was doing a shoot, at iso 50, which most times is very clean even with a 1.5 stop push, and on image review, I noticed that the images were all very noisy. While reviewing, I noted that my 260 was very warm to the touch. At that time I was doing a low/slow (I can't remember which term is used) to the back. I stopped this, and quit my shooting for 10 minutes, allowed the back to cool off, and then image quality returned.
On my 260, USB3 port is a bit tricky, in that if I happen put any movement on where the cable is inserted to the back, I can get a error, and no connection message.
I use a 6 foot and 9 foot cable in the field.
I only briefly tried the FW800 connection on a Macbookpro 15" and did not do a enough shoots to really see if the speed was equal or greater than USB3.
I do find that the USB3 connection to my S Pro 2 is very quick, so much so that I hate going back to the LCD and waiting on the view to refresh to 100% and back.
I can also see your cable length issue being a problem, however I have found that to be a problem with printers also, as most devices seem to spec at the 9 foot length.
Paul