I don't see a problem with the PCIe interface.
If they can fit 6+ (or seeing some of their others, 24+) fpgas on a board, and find a way to power and cool them. A doublewide 24x FPGA board on a pcie which draws 250watt over a pair of 8pin PCIe power connectors. You can put 2 in a standard motherboard, allowing for 48FPGAs per rig. With current bitstreams that's just under 10GHash/s per rig.
Also consider, PCIe allows for MUCH more bandwidth to the card (which is not important at all for bitcoin). But it's the standard right now for FPGA based HPC accelerators. Meaning if you have a PCIe card which works both for bitcoin, and for doing crypto acceleration, or other HPC applications, you now have decent resale value. Current bitcoin mining solutions aren't really good for much else (maybe as a "play" development board, but not for any real serious applications) because of the low IO count, and poor throughput (and lack of available RAM)
I would love to see a 4chip, 8chip and 24chip card lineup, all PCIe, with decent cooling/power arrangements. and offering pricing under $1/MHash (preferably closer to $0.75/MHash or lower, at least on the higher density cards) Even more appealing would be a doublewide card, that has the comm/control chips on the PCIe card, and power supply connectors enough for 24 FPGAs, and can take up to 4 daughter cards, each holding 6 FPGAs. That would allow for high density, and a nice scalability path, to start with a single module on a card for cheap, and add modules over time. Much more approachable than jumping from smaller modules to a $15K purchase for a mini-rig.
The problem is, if you design for bitcoin then the product instantly becomes pretty much worthless for other computations. Bitcoin is all about density, and it can achieve good density because it has no requirement for high speed (and therefore carefully routed) communications. Nor does it need memory. Other applications want lots of high speed, high bandwidth communications and lots of memory, which is more difficult to design for and tends to decrease density.
I think this single slot x1 formfactor is great, and I don't see any reason to use USB when you already have PCIe. Of course it would be nice to have more devices on a card, but very careful attention must be paid to power design.