* Incompatibility with spatial multiplexing. The explanation for this one is definitely best left to a beefy whitepaper, since it requires looking under the hood of how spatial multiplexing in 802.11n really works. The bottom line is that with any commercially practical number of radio chains, it's impossible to achieve the higher data rates in 802.11n and use TxBF at the same time.

* Lots of self-interference. With only 3 or even 4 radio chains to work with, TxBF makes very symmetric beam patterns, generally sending as much energy away from the client of interest as it does toward it. This increases self-interference in the multi-AP networks that are critical to success in today's high-demand-density venues, reducing spectrum re-use and overall system capacity.

* Incompatibility with polarization diversity. There's a technical subtlety at work here, too. The upshot is that TxBF will fail frequently when used with today's mobile clients with arbitrary orientation.

* Modest gains at best. Even when it works, the Wi-Fi chipset engineering community predicts that performance gains in practice will be modest, on the order of 2-3 dB.