A waveguide even at a size of 15" has wide and no controlled directivity below 1000-800 Hz and it beams in the highs. To call that a better directivity is a misunderstanding IMO. The result is coloured reflected energy and an unven response below 1000-800 Hz compared to a CBT. Doesn't the frequency respons below 1000-800 Hz count? Sure, it you have a horn and CD down to schroeder, then it's another comparison and a debate. But that's not the case with the popular waveguides people buy. Most buy either 10" or 12" waveguides. A few bigger ones. The result is both uneven frequency response below the waveguide working area (compared to the CBT) and colouration from the room.
I get the impression that people have become a little obsessed with sidewall reflections within a certain frequency window and are not able to see the whole picture (as described above). Add to the fact that the CBT almost avoids vertical reflections which in almost in every room arrive earlier, are considered more detrimental and gives overall gives less early reflections then the waveguide in many rooms, I think one isn't seing things clearly.
One needs a much bigger horn speaker if you want to even out the odds against CBT. Then you have two different speakers for different applications. Which is better here is more related to the type of use.