The Spirax, Cyclo et al. probably were abandoned due to cost considerations. Spirax must have been very costly to make - it is quite complex. Cyclo and Nivex require a dedicated braze-on on the chainstay to work well. And with any derailleur far ahead of the rear axle, wheel removal can be difficult, so ideally you employ a "repose chaine" and split axle, adding to the complexity.
The plastic Simplex and other were cheaper to buy, easier to install on mass-produced bikes and much easier to set up. With the Cyclo, you need to set the derailleur limit in a trial and error process, then wind the (special) cable around the pulley, join the ends at the shift lever in a tiny barrel with a set screw. The tension of the cable needs to be just right - too tight, and it'll jump out of the set screw, too loose, and it gets awfully sloppy. Ideally, you'll have the Herse or TA eccentric shift lever to take up the cable slack, but that needs to be positioned correctly as well. It's a lot of skilled work compared to slapping on a derailleur, adjusting two limit screws and be done. Which do you think a bike dealer would recommend?
Maybe I'll do a story in my newsletter on how to set up a Cyclo, with photos... First, we'll have the installation of alloy fenders, though.
The Spirax seems similar in setup, but you also need to set the spring tension by rotating the entire derailleur... At least the cable is a single one.
Nivex shifts great, Cyclo shifts well (but there is some slack to consider when an upshift follows a downshift and vice versa - see my story in Rivendell Reader No. 26). Without a return spring on both of these, you don't get any "auto-shifting" because of slipping shift levers and/or frame flex. Once set up, they are wonderful in the hands of a skilled cyclist.
Don't know about other derailleurs. Raymond Henry says the Spirax works well...
Jan Heine, Seattle