When I first started training jiu-jitsu, it was a traditional japanese style which I invested 2.5 years in. During that time, I discovered BJJ because I was looking online for better, more effective techniques than those taught at my school. I left my traditional school to train at a typical BJJ school for a year. That BJJ school had a core group of techniques called the progressive guard, which was what you spent all of your class time learning, except when the instructor was showing a new technique to the whole class. Once the lesson was over, you may never see that technique instructed again. If you missed that class, you would have no idea that the technique even existed.
I have been training at a Gracie Certified Training Center for almost a year now, and I can tell you that some of the people I train with are extremely dedicated to BJJ. At my previous BJJ school, you had two or three 1 hour classes of BJJ per week, that's it. At my Gracie CTC, we routinely put in 3 hours a night, several nights per week. Also, having access to the entire curriculum online is a huge advantage. I can spend as much time as I want developing my understanding of techniques, though I will obviously have to drill them relentlessly afterwards. This is not to say that the GU system is perfect, even for people like me who have access to instructors. There are still flaws in the system, but I wouldn't even consider leaving this system to train at another average BJJ school.
To make a long story short, I agree 100% that two guys learning from GU online and never exposing themselves to other people's styles, techniques, reactions, etc, is better than nothing, but far from perfect. However, there are guys in the GU system who go to every class, dissect every technique, analyze the videos, polish their game tirelessly, and are true jiu-jitsu scientists. These guys win tournaments, gi and no-gi. I think the bottom line is, you get out what you put in.
TLDR: GU isn't just two white belts alone in a basement imitating Rener. Also, effort in = reward out.