Whilst there is a "standard" software interface for Windows video capture devices, which can be built into the Windows driver for that device, AFAIK there is no equivalent "standard" way of communicating with broadcast-video output cards like those from Blackmagic, etc, (i.e. cards which are not PC display cards, which includes the Deltacast card). That means that vMix would have to write specific support for each manufacturer's output cards, not a trivial task, particularly when some cards have very few users, hence the current situation where many cards are not supported.