I plan to buy a new interface in the immediate short term.
Currently on top of the list is the MOTU M4.

However, I am considering buying a hardware effects unit with digital I/Os later.
It would make sense to connect it digitally. The MOTU M4 does not have digital I/Os (SPDIF, AES/EBU, ADAT).
So I was checking out a few other options and have found that there are some cheap interfaces with SPDIF, which I could add as a secondary interface, only for the digital I/O.
The question now is: If I add a cheap interface (like an ESI U24 XL) later just for the SPDIF I/O, does this matter for sound quality?
Initial thought: It is digital. The sound should not change.

On the other hand, people have been discussing wordclocks having a significant effect. If that's the case, this would then mean that digital sound I/Os don't always mean there won't be deterioration of the signal (jitter and stuff).

I am really unsure here.

I am aware of the Focusrite Scarlett interfaces that do have SPDIF. I am not sure wether I would be as happy with a Focusrite Scarlett 8i6 3rd Gen as with the MOTU M4.
There had been complaints about the drivers for Focusrite interfaces in the past. I have been happy in that respect with my MOTU Ultralite. There was a problem at some point which MOTU fixed quickly after I had reported it. Great service.
Many buyers Focusrite interface were not happy at some point.
I have checked a demo on youtube with a Focusrite Scarlett 3rd Gen series' analog I/O. Even with the compression on youtube, the Scarlett didn't compare well.

Be aware that in most cases I'm aware of a DAW will allow you to use only one driver at a time, and you typically get best performance using a specific driver provided by the interface manufacturer. This means that you might not be able to run two interfaces by two different manufacturers - e.g. a MOTU with an RME.

That's an important little detail. Thank you.

According to this document, multiple audio devices can be combined.

https://support.apple.com/en-us/HT202000
A potential drawback could be that drivers from manufacturers can not be used. In case of the well programmed drivers by MOTU, that would be a clear disadvantage.
It also has to be said that theoretic features do not always work in practice.

The good folks at sweetwater write:
https://www.sweetwater.com/sweetcare...or-pc-and-mac/

Yeah Core Audio on Mac and ASIO4ALL on Windows will let you use multiple interfaces simultaneously, but usually not as low latency or stable as the manufacturer's driver (if it even works right at all). It'd be better to have one interface with the connectivity you need. I'm assuming your prospective fx unit would accept analog inputs as well. Not sure if that's feasible or not, but would allow some flexibility for sampling rate on your interface being different from the fx unit (no clue if it's locked at one rate or is selectable).

Right. I was trying to emphasize that if you want to use drivers from a specific manufacturer (you generally should) then you're likely stuck with only the manufacturer's devices, not others.

Sorry if that wasn't clear. I guess I should have added "..if you want to use what is likely the best performing driver" at the end.