I did join SBL at the same time as taking up lessons with a tutor. The face to face lessons cost me way more but with the incentive of a real person who I am paying waiting at the end of the week I worked much harder. Also the lessons were very much personalised for me and I learned saleable skills which are now getting me work.
I used a course on SBL to learn a specific skill and I did. It worked just fine. Yes there is interesting background chat from the tutor just like my real life tutor would chat. I never measured it but 80/20 does seem a little unlikely. Simple fact is you can't just teach teach teach you need to allow a student to absorb the lesson as you go along. Learning for me is an holistic experience so, for example, the slap course for beginners included some information about the history of slap bass which I found valuable and informative.
Having said all that, don't fecking charge me for a course if I'm already a payed up member. Leaves a very bad taste.