In the entire pipeline, which part do you find most time-consuming or error-prone? Do you have any advice for beginners to avoid common mistakes in that part?

This is a compatibility issue with the Metal backend in iOS 15. Apple has a history of changing shader handling without much documentation, and that affects older frameworks like B4W. Based on the 'INVALID_OPERATION' error when calling gl.linkProgram, there may be a change in how the system checks Bitlife or optimizes shaders before linking. I recommend breaking the shader down, testing each part individually, and using EXT_debug_shaders if possible. Alternatively, you could try implementing a simpler fallback shader to see if the issue is coming from a specific feature.