What is advanced yoga? Is it the height of your leg in bird of paradise pose, or the height of your consciousness? I think it's both. There's a lot of debate now about what constitutes 'real' yoga; with one school calling what a lot of current yogis on instagram do 'gymnastics', and the other school well... they're just happily carrying on with their handstands. Fact is, one of the main tenets of yoga is to be aware of and then diminish the ego, and if your practice is advanced enough that you have mastered the ego, you shouldn't need to worry about what anyone else is doing on their mat.
The problem with reducing advanced yoga to simply asana is that you are immediately removing a vast majority of the population from the 'advanced' equation. It makes it into an exclusive club, the entry to which often requires a supremely hypermobile body and, yes, a background in gymnastics. Yoga literally means 'to yoke', or to bring together, to unify. You can't advance the unity of a certain population if there is only one way to get there. There is also the very real issue of cultural erasure, when you only take the elements you like from a tradition steeped in literally thousands of years of rich and complex history. Advanced asana is great, and it's even better when presented within the vast tapestry that makes up yoga.
I think attempting poses that present a degree of fear and the risk of confronting the shadow of your ego are important. Poses that are physically and mentally demanding increase your threshold for stress, meaning that when you encounter stress away from you yoga mat, you are most likely better equipped to deal with it. Sometimes people write off a certain yoga pose before they have even tried it. This could very well be down to physical limitation by way of injury etc, and in those cases I am a strong advocate for safe practice, but if the limitation is purely in your mind and therefore your ego, I think getting into your body in a way that demands a deepening of your relationship to it is essential. You do not have to immediately do the photo-perfect pose, or even get close, but it's what you discover upon the approach that is the real yoga. A common misconception in yoga is that it is all about feeling at ease all the time and stretching out the body and bypassing all challenge on the way.
When I teach I talk about Patanjali's concept of skill-in-motion (stira/sukha), the constant search for balance between effort and ease. What matters to me is not the shape of your physical body in a pose, but the quality of your subtle being when you are doing it. How can I see a person's subtle form? Well, I can't quite, but the closest physical representation of it is the breath. Subtle energy or 'prana' is contained in the breath, it is our life force. If, in a complex physical posture I can see you are putting the limbs in their various places that is one thing, but I want to know how you breathe within a pose. How long can you stay still in it? This isn't so much in a competitive sense, but rather how much ease can you find whilst using your best efforts?
Advanced yoga is many different things to many different people. Some of the most dedicated yogis I know and have had the pleasure of meeting haven't set foot in a yoga studio. In a way, there is no advanced yoga, because it is an ongoing practice, with no end goal.