Ribbon midrange pros and cons


Ribbon tweeters are fairly common on some high end speakers. ...they sound great.....can anyone tell me why ribbon midranges are hardly ever used by some of the best speaker companies. ..what are the pros and cons? 
128x128vinnydabully
RAAL makes ribbon tweeters  
  
Companies like GoldenEar use AMT tweeters, they look similar but are different tech.  
  
The RAAL tweeters are some of the best on the market, but they don’t make any midranges, and I don’t think I have ever seen a ribbon midrange, probably because those frequencies require much more air movement. 
@vinnydabully 
Not really sure why we refer to some form of Ribbons as not true ribbons, whether its an open (dipole)design or backed (which causes cancellation) or a folded ribbon (AMT), it seems to me that it is still a ribbon.  As far as strengths and weaknesses, I believe that many have been addressed. One which can be both is speed.  Ribbons are fast, There have been several companies that produced ribbon speakers that when used with a tradional cone woofer, mid woofer or sub,  had issues with the speed of the ribbon matching the speed of the cone driver.  Next,  it was mentioned that a ribbon without some sort of transformer was a problem. The amplifier sees the ribbon with such a low resistance that it is basically an open load and will cause most amplifiers to have a real fit. Never hook a ribbon driver to an amplifer without a crossover in front of it. Directivity is effected by ribbon length,  so combining directivity with panels that are dipoles, it is not always easy to get optimum set up in many rooms. Ribbons done right can be a revealing experience.  I do recommend everyone to sit in front of some very good ribbons at some time in their audiophile walk.  They are not for everyone, but when done right and well set up, they can be a great experience. 
I hope this helps, 
Tim 
Correct, Raal tweeters are fantastic and the sound coming out is also dependant on the overall speaker design. Just cannot say all ribbons sound bad as a blanket statement. Just not true. Robert of Acoustic Zen incorporates a ribbon in his Crescendo speakers that sound wonderful. Again, the complete design is key! Tonian Labs also uses ribbons and they are some of the most musical speakers I have listened to. 
Thank you guys for all the responses. .those explanations were on the same lines as l was thinking ...l was particularly thinking of the infinity Beta ,the Epsilon and the new speakers PS Audio is working on...Paul says hes working on a 120 db. Ribbon midrange .l just dont see other manufacturers using these types of midranges and wondering why.l an going to Expona this weekend in Chicago to listen to some great systems. ..lm sure to be impressed. ...