Gaz Williams pointed out on Sonic Talk 501 (36:50) that you can use the stereo Out A as effect send and then use one of the stereo inputs as a return. (Unfortunately there are only two stereo channels and they double as computer USB return channels.) They also discussed whether it might be possible to use one of the five headphone mixes for a send – i’m not sure how well that would work?
There are also the 16 built-in effects (page 95 of manual).
Still think this looks like an interesting product, depending on how good it actually sounds. It definitely can do a lot for $600.
Added: Also discussed in Sonic Talk 501 (toward the end) is the iOS Kymatica AUM mixer with a XR-18 or even potentially with the L-12. Sounds like a great combination.