The tolerance for interference fit ("clutch power" in Lego terminology) is important, but that's fairly simple. It's the cumulative tolerance when you assemble large structures that's important. Knockoff bricks can be fine for the first few you assemble, and then as the structure gets larger things don't quite fit together.

Also interesting is that in very large models, there is decoupling between sections. Lego has design rules for how large a well connected chunk of Lego can be, which are driven by the tolerances. Above that you are then loosely coupling those large "chunks".

That's actually not too difficult. So long as your process variance is centered around nominal, the stackup will tend to cancel out. You might run into trouble if your kit involves hundreds of identical pieces from the same batch being assembled together, but that's rare. For large builds from multiple kits, it's very unlikely they have the same errors.

This is the most interesting point in the thread to me. Tolerance stack-up is the reason tight per-part tolerances matter at all. A single brick being precise is table stakes for injection molding. The hard problem is what happens when you compose hundreds of them. The decoupling strategy you're describing is really similar to how you handle error accumulation in any large composed system. You can't make individual components perfect enough to avoid drift at scale, so you introduce boundaries where the accumulated error gets absorbed rather than propagated. In Lego's case that means designing joints between sections that are forgiving enough to accommodate the stack-up from each chunk independently.

It's also why knockoff bricks can feel fine for small builds and then fall apart (sometimes literally) on larger ones. If your per-part tolerance is 3x worse, it doesn't matter much for a 20-piece build, but for a 2000-piece build your cumulative error budget is blown long before you're done. The failure mode isn't that any individual brick is bad, it's that the composition doesn't hold.

I'd be curious whether Lego publishes or talks about those chunk size design rules anywhere. That seems like the actually interesting engineering story, more so than the per-part tolerance numbers that get repeated in every article about them.