Basically you have a much more limited choice of qulaity tweeters. With domes you have to use dayton and a very small others of the 3/4 inch size, and then you have to use a smaller midrange, usually not bigger than 5 inch. With planars and ribbons you have more choice, but if you use a 6.5 or 7 inch woofer, you already know that you will have a limited number of tweeters that go low enough without using 8 db/octave crossovers, that you use.

ITS all a balancing act with line arrays due to the comb filter distortion issues.

Marlboro