I occasionally take inspiration for what I write from misunderstandings which arise on martial arts forums, because they are often a reflection of widely held misconceptions. Unfortunately, such discussions have a habit of descending into juvenile name calling and finger pointing: the result is that no one comes out of it looking good.
In this particular instance, an argument over how much Wing Chun was actually taught to Bruce Lee led to someone - claiming to be a Wing Chun student - arguing that second form Chum Kiu is a more advanced form than the third form of Biu Jee. As in any other martial art where forms/taolu/kata/hyung/jurus/anyo are taught, such an argument is nonsensical.
If we were to consider the whole system of Wing Chun as a jigsaw, each form, whether empty hand, dummy or weapon, gives us a certain number of pieces of that jigsaw. Sil Lum Tao, in this respect, could be said to give us the first pieces and, if those pieces were missing, we could never see the finished picture. The same can be said of the other forms: if those pieces are missing, you do not have the full picture. So, rather than the form itself being advanced, it is the progress of the student which has advanced, when they are ready to learn another series of movements.
As for Bruce Lee, he did not have all the pieces of the jigsaw, as far as Wing Chun was concerned. Instead, he used those pieces he already had to create a new jigsaw - a concept he called Jeet Kune Do. The analogy is getting rather thin now but, put simply, he recognised the gaps in his knowledge and went outside of Wing Chun to fill those gaps. I'm not here to comment on how successful he was in completing his knowledge from other sources, and certainly have no interest in art A versus art B conversations.
It is only through learning a system that you can know which pieces of it are useful to you. If you do not have all those pieces, it is easy to think that what you have is worthless, whereas those pieces may actually be of utmost importance to someone else.
So, there are no advanced forms, no advanced techniques: it must all work together. Each set of pieces must join with those by which they were preceded.