Played around with tally over NDI the other day with some birddog convertors.
While it works nice, there are a few issues.
First, sometimes there is a slight delay of about 1 second maximum before the tally light responds. When you cut away, this is not a problem.
It might be before you cut to a source. And even more so because of the way mimolive handles the preview tally. If a layer is selected but not live it sends a preview signal. That means that you lose the proper preview tally if you need to make changes to another layer, since you need to elect that other layer.
And if you are in the video switcher layer, you only get a program tally. It would great if preview functionality was added here. And maybe on the layers with a send button that can be toggled on or off.
While we are on the subject of tally, do you know cuebi? These small tally lights operate via ip over wifi, are cheap and work great. Integrating these would be awesome.