I just discovered the Thunderbird add-on CardBook which seems, so far, to work far better than SoGo Connector -- which I think is responsible for most of the difficulties I've had with CardDAV so far.
It synced the expected nr of address records in the case of my Baikal address book, my copy of my wife's and the intended shared addressbook, as well as others.
I came across a passing reference to CardBook on Fastmail's site recently, which was the first I'd heard of it, and I just got around to trying it out. The reviews on Mozilla's site seem very largely positive and others are using it with Baikal, apparently with good results.
At a minimum I suggest adding a link to it to the docs and updating the info on SoGo Connector (latest is 31.0.3) to include the constraint on server addresses.