Glare and shout are the problem....well, that and beam if you're not far enough away from them.
Most of the problems about glare and shout in horns come from bad set ups: Compression drivers inside horns have such a dinamic rage and such a low distortion due to the diaphragms very small movement, that it is very easy to set the volume of the horn at an extremely high volume in comparison to the accompanying channels, ussually from 800 hz down. For comparison sake: if you had a bass system as dinamic as the horns playing at the same volume you will be blowing your windows of the wall! But most horn systems in use today have a single 15" woofer covering the most difficult range from 800hz down to 40-30 hz, this driver, as good as some are (Altec), is used exceedingly beyond its reach.
Using big horns to cover from 100-120 hz up to 800 hz and then the compression driver of choice from say 800 hz up to 12khz etc, will let you listen what horns are really all about! If you never listened to a system like this, you probably dont know what horns really sound like...
About glare and shout there is a driver-horn combination also that has to be used to avoid this: Tests have to be run to avoid glare and shout and it can totally dissapear: For example, round tactrix horns, I found dont like Radian 950 drivers (nice drivers mind you), in this type of horns they do become shouty, it can be fixed using 20 ga copper wire instead of fat high end cables, but some penalties occur in low level detail loss: Same thing for certain TAD drivers. Maybe Exponential curved horns or other profiles will help here. I go for tactrix profile mainly because its size, and because they are so good at near field listening: which bring us to the next point in the prior quote: Horns can be used for nearfield listening if well designed and set up, detail and soundstage becomes expansive. Volume has to be carefully put taking in account for the wonderfull dynamics horns can achieve, if you start with a high volume for the lowest pianissimo passages, when the crescendo comes in it will be powerful!
About xovering, horns have a natural slope and after the horn cut off; a relation to its size, response drops faster than a 24 db filter! So in certain aspects xovers can be skipped, but sound does suffer a little in transparency, a simple 6 db xover following the horn natural cut off will do wonders, and if put before the amplifier and you multiamp with good SETs, well heaven is right around the corner!