I'll clarify (and repeat) the show-stopper issue for the 550 in the US:
Ubiquiti chose to only have it type approved with low-gain antennas. That limits the gain that can leagally be used on any antenna. They could have filed a test with a 30 dB or more antenna, but chose not to. The FCC rules allow unlimited antenna gain; on 5.8 GHz, only out of band emission is limited, not in-band EIRP. So Ubiquiti cripped the radio.
Of course FPGA radios cost more. But charging for every little speed step, encryption, etc., as the 650/670 line does, puts it into a different category. It's sold to customers who are absolutely insensitive to price. I've seen a lot in public sector applications.