ES-8 works great as an ADAT expander, I have one permanently connected to my audio interface and only use it in USB mode rarely. The Optx does have a channel advantage, but if sending CV into your DAW the ES-8 has DC coupled inputs as well as outputs.
I’m not sure if the Motu Ultralite mk3 does have ADAT though, so def worth double checking that. If confirmed that it does not, (and you’re using a Mac), the ES-8 will have the advantage of being able to be set up as an aggregate audio interface (this doesn’t work so well on Windows).