Most already know this, but it cannot be overemphasized:
Rear derailleurs shift very differently with different freewheels and chains. I used to run a Campy Rally (2nd version with cast upper piece) with a 13-26 Dura Ace 7-sp freewheel and Sedis chain. Worked like a dream, and I wondered why everybody was maligning the poor Rally derailleur.
Then I decided to try half-step, so on went a 14-28 Regina Oro 5 or 6-sp (don't remember which) with Regina 5/6-speed chain. And it didn't shift. Every change was a struggle, accompanied with embarassing noises... (So this must have been the setup that Frank Berto tested in Bicycling! magazine and found lacking.)
Now the bike has a Regina Extra 5-speed, back to Sedis chain and Huret Duopar derailleur. Shifts great. (Only the freewheels keep breaking - the cogs of my batch of Extras don't seem strong enough. Manufacturing defect, or are hill intervals with a 70 lb. trailer just too much of a load, especially when using a 24-tooth front ring?)
The amazing thing about some of the old derailleurs - Nivex and Duopar, for example - is that they work well with older freewheels and wide, stiff chains.
However, most old derailleurs (my experience is limited to Campy NR/SR/Rally and Simplex) work very well when coupled with newer (but still in the timeline!) freewheels (especially Shimano) and chains.
Jan Heine, Seattle (who considers the freewheels the only worth while part Shimano ever has produced! But that is just a personal opinion...)