It appears, that Analogue Systems RS170 multiple can be used as a unity
gain mixer, while Doepfer A-180 cannot. What is the reason for this?
RS170 looks a better buy...

From RS170 manual:
"On other occasions, you may wish to mix signals before presenting them
to a single input. If you wish to control the proportions in which this
is done, you should use an RS160 or RS165 Mixer. If you simply wish to
sum the signals, you can use the RS170 as a pair of passive 4-channel
mixers with unity gain."

From Doepfer forum:
"...In principle an A-180 cannot do this job [Mix 2 control voltages]
as you never should short outputs (but that's what you do if you
connect the CV out of an A-190 and A-155). In the A-100 nothing can be
damaged by shortening outputs but it's a basic principle not to do that.
For your application you need a mixer/adder..."