To me, a HUGE factor in riding an ADV bike off-road is know the limitations of your skill, same for your bike, and being able to read terrain. If you can pair up with a superior rider and get lots of seat time in, you'll make a lot of headway. Until a rider can learn to relax on the bike, many situations will continue to be more difficult. Once you get comfortable with the bike moving around under you, you'll really start to improve.
I'm a 30+ year self taught dirtbike guy turned ADV'r, but a few years back I took the Shane Watts Dirtwise class. He confirmed that I was doing some things well, but being self taught, had some bad habits too. So, even a long time experienced rider can improved from class time.
A lot of my dirt skills mapped over (controls, line selection, balance) but riding a 500lb. bike is very different from a 250lb. dirtbike. So, it's been cool learning how to ride my bike well.
Training is great and you should do it, but there is no substitute for seat time IMHO.