Horn speakers are really bright?

Hello lordrootman.  Many horn drivers used  for mid-range and tweeters are very efficient: 95 db+. Most woofers are not nearly that sensitive. If someone tries to pair a sensitive horn tweeter with an average woofer, an unsatisfactory result is likely.  Always try to match the sensitivities of the drivers in a multi-speaker package. That way, you will not have to use resistors in the crossover to reduce tweeter sensitivities. Resistors waste power and prevent the amplifier from accurately compensating for speaker errors (reduces the damping factor). A mismatch in driver efficiencies generally results in a harsh of screechy sound. It's not the drivers fault! Horn tweeters using non-peizo compresson drivers generally have large magnets and can provide excellent performance at reasonable cost. Use plastic horns (vs. metal) to avoid ringing. You can coat the outer (rear) surface of the horns with auto undercoating to make them acoustically inert. The early AMT tweeters (still being sold) have large metal surfaces (part of the magnet structure) exposed to the sound coming from the diaphragm. I glue tea towel or sock material (cotton) to those surfaces to knock out the resonances caused by the reflection off those surfaces. Huge improvement! Use silicon rubber and you can always remove it easily if you experiments don't work out well. Experiment, Enjoy the music!

My friend who a DIY speakers builder come to conclusion that compression drivers with metal diaphragms are so transparent and detailed and it shows imperfection of electronics.

One example from my experience, when I just bought 300B amplifier I used it with Spendor 2/3 and Altec 604E speakers. With Spendors sound was good but with Altec was some harshness. In the end the reason was bad (harsh sounding) cheap 6f6 drivers tube. When I replaced them to better 6f6 harshness on Altecs was gone. But on other hand, Spendors didn't sound harsh even with the bad 6f6!

So much depends on how loud you drive them.  Some people's "loud" is NOT loud to me. My loud is VERY loud and that's where the horns get shouty and ear piercing.  At the same volume, non-horn speakers like my Tektons are much more pleasant to rock to.  I've thrown many a party with the Cornwall and CF3... They certainly deliver but then you get to something like Celine Dions voice and you have to leave the room because it's very sharp.

Horn speakers they depict as sounding dynamic and live .
i went out of my way to hear fleetwood deville speakers at a audio get together 
they sound nice dynamic and very live as in real ,But they are more first few rows perspective ,in my opinion over time in your face even though not bright 
can be tiresome ,unless you have tubes to mellow it out .i have very good solid state that is smooth , such a shame too for they look very nice.
They should offer Xover parts options such as Duelund ,Jupiters Vh audio Milflex With top Mundorf Copper foil resistors, or Path audio.
peach having their own perspectives ,and from my vast experience with Xovers 
pit would Bree more natural and take on a much more friendly 7-8 th row perspective.. this is the problem with companies that think they know it all when in fact they know so little about Xover parts sonics. I am basing this on over 20 years on most every thing out there , being a long time Klipsch guy who has rebuilt many and figured out how to control their sonics to several Sonic perspectives.

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...