ELI5?

I see a lot of critique in the previous (2019) thread, but no summary in key points. What are the reasons that this is hard?

In multiprocessor systems, are memory writes not guaranteed to be sequential? e.g. can data be written out-of-order, after the synchronization bit is written?

Or is it more about the use case, that it is optimized for minimal number of instructions, e.g. avoiding additional memory reads? (e.g. by writing data to the buffer, instead writing storing pointers)?

Or is it that you're trying to use constant memory (irrespective of number of threads)?

Because to me, it seems like a trivial problem to solve, if you have sequential writes, store data separately from the queue, and may linearly scale memory with number of threads.