Only drawbacks I see with selecting the UC(X) are outputs 7/8 will be unbalanced from the Stereo HP Jack on the front. If that is not an issue, I would also recommend the UC(X) as you will have a second computer interface for redundancy should one fail.
The ADI-8 is strictly a standalone AD/DA converter with no computer connectivity options, but might "spec out" a little better overall (likely inaudible to 99.99% of users)? The ADI-8 also allows DB25 connections and all I/O is identical/symmetrical - so that can be a plus when interfacing with an analog mixer. Connection to UC(X) would be over ADAT - and since the UC(X) only has a single ADAT - you are limited to 44/48K for all 8 channels, or if using 88/96K you will only have 4 channels of ADAT I/O from ADI-8 to UC(X)...
If you go with a second UC(X), I would recommend using only one UCX to the PC, and then connect the second UCX to the first over ADAT (standalone mode - just like the ADI-8). This will allow a single instance of TMFX to route all signals instead of having two separate TMFX mixers to manage (but two TM's would net you additional TMFX DSP power if needed). 44/48K ADAT restrictions also apply here - BUT you could also choose to connect both UC(X)'s to your PC (with proper USB/FW controller provisioning) and get 96K-192K across all 16 I/O's (both units). Using the same driver for both means any DAW should handle that setup - but you will have 2x TMFX's to wrangle as needed...
Pros and cons to each - Up to you to decode which will fit your workflow best
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8