The card does have a mixer. I have never used a card with a mixer, so that will be enlightening.
I assume the only place where there will be latency is the interface between Cubase and the card.
What is the best way to measure and handle this latency? I know Cubase Pro handles this beautifully via External FX. Is that the preferred way of interfacing Cubase to hardware?
Latency issues would be an issue under several scenarios.
Your issue will be that you have to manually adjust the delay compensation for the returned track in relation to other tracks in the project. This can create some monitoring challenges depending on buffer settings of the card, size of project and what the total latency is for other tracks in the project.
The only difference is that in Pro it does the calculation for you. And, it comes with the penalty of tying up the outputs for a given project. Calculating the offset isn’t that difficult. But, there are so many other advantages to Pro it would probably be worth it.
But it’s important to understand that Pro does NOT change the latency. It simply adjusts playback to align to the recorded latency. If you are trying to play and monitor, you will notice.