Jan-Martin wrote, "perhaps not so much for RF, but I wonder why you
don't see NPN/PNP combination for darlington transistors - with Vbe =
0.55-0.6V"

Actually, I think if you look at schematics of transistorized audio
amplifiers, you'll find examples of about every practical combination
of two and three transistors, used to build complimentary output
stages. If I understand correctly what you are suggesting, I'm quite
sure I've seen that configuration. In the 1960's and into the '70's,
there were a lot of games played to try to pair up NPN power
transistors with PNP drivers to get PNP-equivalents and NPN-equivalents
that were as nearly symmetrical as possible while giving high current
gain, since PNP power transistors weren't up to performing as well as
the NPNs in that era.

Cheers,
Tom