Are Horn Speakers good or bad or simply a complete joke?

Just FWIW... to get horns and other high efficiency speakers to sound right usually involves an amplifier with a relatively high impedance output. Nelson Pass published an excellent article that raised this issue in Audio Express around the beginning of this year.

High efficiency speaker tend to be highly reactive and do not take kindly to overdamping! So the best sound is usually with an amplifier of higher output impedance. This, combined with the fact that tubes sound better then transistors anyway, is why SETs and OTLs are the best amps for horns. Just about any other combination will get you shrill results due to the reactive nature of the speaker.

IOW, horns are not shrill by nature. They get this reputation from being used with the wrong equipment!

Not at all. Horns, like cone speakers, come in a variety of performance capabilities. IOW, some work very well, are highly revealing and others pretty much suck at the same task- just like cone or for that matter planar speakers. It would be throwing out the baby with the bath to assume that just because one horn speaker sounds terrible, that they all do!