Horn based loudspeakers why the controversy?

Horns sound more like real musicians than conventional speakers, to me at least.

 You cannot get the same dynamic range with inefficient speakers. You can apply as much power as you want but you will not get as great a range between the quiet and the loud as you will with a more efficient speaker. 
That is why typically efficient speakers will sound more realistic at lower volumes.

I'm a fan of horns, though I currently don't run a pair.

I have some close friends who REALLY dislike horns, and instead of trying to convince or talk over them, I prefer to listen and understand their complaints.  Beyond the typical complaints about shout, some centers on the integration or lack thereof of such disparate drivers.  Even as a fan, I acknowledge and understand, though I've listened to more than a few "regular" designs that do as badly or worse.

One more thing...horns come in both front-loaded and back-loaded configurations.  Almost always in these discussions, we mean the former when we use the terms generically, but some excellent back-loaded horn products have come down the pike