Horn speakers are really bright?

While I cannot comment on commercial / lower grade horns, as I have not heard any, my reference is the four horn systems that my friends and I have here on the island. They are in systems that were fine tuned for decades, by the most hard-core audiophiles who have tried everything in their life (cost no object), and settled with horns. Two are Altec A5-based (with crossovers modified to Valhalla and beyond), one is a big Edgar horn (with the Edgarhorn sub!), and one is my own design from ground up (Altec 515/288 drivers).
A common feature in these systems is there is not a hint of "horn coloration" in any of these sounds. If you were blind folded, you would not be able to tell from the tonality that you are listening to horns (zero honk / shout), although you could immediately tell by the shocking naturalness, lack of audiofile "plastic processed sound", lightning speed, full size, dense substance, and lack of even a hint of compression or listening fatigue at any level that these are unlike any speakers you heard. You can play these at +20dB SPL compared to other speaker technologies and have less listening fatigue than with others at that much lower SPLs.

They all give an experience that totally messes with you, and makes you feel that these are technologies from another planet compared to mainstream stereo gear.

Horn speakers are a life-long quest. Do not even think to have them if there are problems in your chain elsewhere, as they will show without mercy when the emperor is naked. And most audio gear is definitively unflattering when there's nowhere to hide.

If you are curious about horns, try to find someone in your area who has done it to perfection, and listen to it. You may not be lucky to have anyone near you though.... or you might have several. YMMV...

What island are you living on?

The strength of a top notch horn system is the ability to sound great at very low volume levels--they still manage to sound lively, detailed and engaging even when played softly.  Some systems do tend toward "shouting" at extremely high volume levels, but, I hardly care about that because I don't like to listen at that kind of level.  Moreover, the very best amps, to me, are all low in power and can deliver great sound without having to "raise the roof."  I don't care if a system is unable to achieve rock concert listening levels; if my system ever sounded as bad a rock concert, it would be gone the next day.

then you get to something like Celine Dions voice and you have to leave the room because it's very sharp.

Not sure I'd want to be at that party...........

Well listening to Tri of Triplanars field coil horn system certainly enlightened me to the strengths of horns as have a few other systems I have heard throughout the years. Exposure to these better horn speakers had given me a appreciation of this type.  Low level detail is one of the greatest strengths of horns.

I do feel however that many horn users underpower their speakers and this has nothing to do with volume just some of the intrinsic problems present with SET amps. If I were to go horn I would use OTLs, which are the amps I use on my dynamic speakers currently. 

I have yet to hear a dynamic woofer integrate well with horns. I have only heard horn loaded woofers a few times and this was really special, but this didnt change what I dont like about horns just made the system more cohesive.