[AussieWog93]

I've written a device and driver side USB stack, not even bare metal and it took several weeks of attempts and hundreds of pages of reading (mostly USB complete by Jan Axelson, this site is also good but just not as in-depth) to get right and there are still parts of it I don't fully understand.

Call it 15 hours to get a "ping" signal across, and another 15 to get data travelling at full speed with proper SOF handling.

Nowhere near as simple as something like I2C that you can get a complete understanding of in an afternoon.

[CableNinja]

Its even more complicated now. USB4 has a lot of timing related things that the system has to do before you can even start sending data. More complicated, much of it is done in hardware, which might be nice for quickly slapping USB capabilities, buttttt is a bear if you're trying to build something thats not a computer. Theres cases where the device wont even power on unless you do everything just right hardware side. Gone are the days of simple signals and connections.

USB isnt so universal anymore