Hi Heim,
We have little experience on the Mac products therefore we can provide you some feedback based on our internal testing on the Intel HD 4000 chipset which you have on your Mac Mini.
Test results on the HD 4000 on the CSV-5300A it does output all at 1080p on 3 displays but unfortunately it acts as a second display, it will not work on 3 independent 1080p displays.
I think I can blame this to the driver support.
On the HD5000 series we have found that it works well supporting 3 independent displays at 1080p.
On both cases the test was done on the Intel NUC with Intel HD 4000 graphics and HD5000 graphics.

At hardwareluxx in Germany they tested our Y-Cabled Docking station which uses the same chipset as the CSV-5300A, the STDP 4320 and they were able to drive two displays without issues with the Intel Graphics Iris Pro.

You are absolutely right about the Matrox in regards of the MST Hub. The MST can always drive displays from your graphics source, it does not became obsolete as it does not have an independent graphics chipset in a box.

I hope this information help you to make your own decision.

Regards