Some futher testing revealed that with the current version of GotoMeeting on OS X, the issue seems to be anything that does not deliver 25fps. Currently I got my Atem Mini Pro running with GotoMeeting at 1080p 25fps, let's see for how long. Yesterday I had a complete crash of the program.
I tried the RGBLink Mini, the obvious competitor to the Atem Mini (without any pro) and it also does not work with GotoMeeting at 720p since it cannot do 25fps at 720p, only at 1080p. However, it can flip preview/program with USB-UVC and HDMI out, so one can use a compatible HDMI-Grabber such as the cheap ones from Avermedia, or Elgato. (It seems to work at 1080p 25fps, too. But same issue as with above, would it fail me during the show?)
Please, folks at Blackmagic, make the RGBLink what it should be: the device inferior to the Atem Mini.

how? Add 720p output, and allow to send preview to USB-UVC and program to HDMI.
Well, actually one must also admit that it is GotoMeeting that is a PITA here, but we all need to feed the video conference tool our clients believe they must use.
Bad: It turns out GotoMeeting somehow can use a 1080p signal but it does not scale it down, it crops it in. And also on OS X at least I can't prevent the image from being mirrored.