I'm actually glad to see that restriction in their study, because there are many cases where you just can't / more difficult to use PGO. One of the most common reasons I ran into is that clients refuse to use it (no, I'm not even joking), either they don't know how to come up with a good training data (in which case they'll point at your nose shouting IT DOESN'T WORK!!) or they thought it's a stupid idea (again, not joking). You'll be amazed by some people's stubbornness