The short story is they cause a lot of co-channel interference and have a very high noise floor.

The longer version is they don't play well with others, at least none of the ones I've seen have. On 2.4 GHz they use channels outside of the standard 1, 6 and 11 (in the US) and are configured for 40 MHz wide bandwidth, neither of which should ever happen. 5GHz behavior is a little more acceptable because there are more non-overlapping channels available, but they tend to migrate to the busiest channels (UNII-1, UNII-3) instead of going to less-occupied spectrum (UNII-2, UNII-2e). They are taking advantage of the new B regulations allowing higher Tx, but I question if they're actually following regulations.