Another level of support is the number of monitoring buses that can be used for ASIO direct monitoring.
Some interfaces or software are limited to a single stereo bus.
If you watch at AVID, full direct monitoring support is only available on HDX systems, other systems, even the HD ones, are limited to a single monitoring bus.
In a pro studio setup, it is desirable to have more than one monitoring bus. It is not uncommon to use two or three separate cue buses for performers headphones, so they can get different mix levels for their comfort.
Other systems (seems to be the case for the MR816x or MR816csx), when more than one interface is used, cannot daisy chain the direct monitoring signal to the interface where the monitoring bus is located, seriously degrading the direct monitoring usefulness for tracking sessions with more inputs that the input count of the interface where the monitoring bus is located.
In practice, with Steinberg hardware, this is limiting the number of direct monitor inputs (at 96 KHz sampling frequency) to 12 inputs (MR816) or 16 inputs UR824.
Or 16 inputs and 24 inputs at 48 KHz sampling frequency. (the difference does come from the ADAT ports than can only get 4 channels at 96 KHz
It seems that there are very few manufacturers and interfaces with full low latency monitoring support, and most if not all of them are pro expensive interfaces or networked audio systems.
Those interface are often those that fall in the real pro category, with +4dBu inputs and outputs at -18 dBFS (at least + 22 dB of headroom).
If you watch at the market, all mid end interfaces, even RME ones for examples, are limited to 18 or 19 dB headroom (+4dBU at -14 dBFS). This mean that they cannot be seriously integrated in a professional studio setup, where other hardware is using -18dBFS most of the time.
So those two capabilities, full direct monitoring support and true +4dBu support are segmenting the market between consumer and pro level interfaces.
Last, there is at least one manufacturer that try to push to the market a large channel count Thunderbolt interface without DSP. This mean no interface direct monitoring capability. The monitoring is always done through the DAW.
If their low latency performance is ok in the order of the millisecond, i'm septic about the possibility to use them at such a low buffer setting, as soon as there is a few effects and a few channels in the DAW mixer, something quite common when tracking a band...
The conclusion is that the direct monitoring capability, when fully implemented, rise the price of the hardware, a lot... But when you need it, you need it.