If the asanas are all easy to perform in the beginning this can give the illusion as if the method of practicing yoga is understood. This can be the case, needn't be so. Simply being able to twist or being able to backband is only one part of the whole picture.
Those who struggle with asanas in the beginning have to find ways to learn it. It's likely that they discover how the breath can help to improve the asanas. With an inhaling the body can be lengthened, with an exhaling one is able to go deeper into an asana. Engaging the bandhas might also help to improve an asana. Appling these methods help when much more challenging asanas come.
The learning process might be faster after some years for those who struggled in the beginning, while those who have it easy in the beginning might have to get back to the basic work and have to learn matching the breath with the movement, engaging the bandhas and the muscles, remembering the drishtis.
If the process is slow in the beginning it becomes very fast clear that patience must be learned. This is necessary for the more advanced asanas. Patience can be learned.
I'm still absolutely in awe when I see what my body is able to do, I love to see others and myself in crazy body postures. Lately I'm equally fascinated by the mind. It can be seen if someone has focus or not. This gives intensity to a practice.
You surely have seen it already: an actor is on a huge stage and the whole room is dominated by his/her presence. "He has presence", the spectators use to say: This is what focus can do.
Being strong, flexible, healthy is the one side of the coin, the other is the mind, the ability to control the mind. It's as difficult as back bending......