Synth, string, organ and wind instrument patches do not sustain correctly.
Some instruments naturally do not sustain - guitar or xylophone, for example. This is handled correctly.
However some instruments should *always* sustain forever, as long as the note is held. Synth, strings, organs and wind instruments, for example - all these should *never* stop playing the note until you actually release the note. The YRG is wrong on these, because they all decay after about 5s or so, even if you keep holding the note down (and the orange LEDs report that the note is still held). Sorry to be so absolute, but anything other than infinite sustain is plain wrong when compared to 2300 years of organs, 1400 years of stringed instruments or an indefinite history of circular breathing techniques.
This is particularly a problem when you're using the YRG as a synth. Pads simply don't work properly if you can't keep them held on.
I would have raised this as a tech support issue, but I've already hit my limit for tech support tickets!