The Dugan automixer is made for situations where you want mostly one sound source at a time, so one should think of it more of an auto selector. A choir is very much the opposite of that - you need all voices to be heard simultaneously to achieve harmony, so I'd say the Dugan is very much not what you want to use on choir mics.
I once accidentally recorded all radio mics in a musical theater production through the Dugan mixer (= with the Dugan inserted on all channels) for later virtual sound checking/tweaking and the choir/ensemble parts didn't work at all - which was how I discovered that I'd recorded through the Dugan, but it took me a while. I couldn't really that that was the problem at first (no audible artifacts etc.) but it just did not glue at all like it should/did live.
