Shouldn't be more difficult than other fretless string instruments, which is most.
Like the Cello for instance:
http://www.youtube.com/watch?v=09RUuTAM2H0
The cello is not really a chording instrument. It can be polyphonic but that's not really how it's used/meant to be used, which is because/why it's fretless.
Achieving good intonation with a fretless fingerboard is one of the primary challenges with playing an instrument from the violin family... and it's genuinely difficult, which is why most student violinists sound awful.
Really I don't understand the point in this other than to say 'hey look what I can do' as there's not a whole lot more than you can achieve with something like this than is already done regularly with existing techniques, and it cuts out like 79% of the usefulness of the guitar.
It does seem pretty cool with an ebow though
