usb is kind of buttcheecks too though. Everything is much cheaper and off the shelf I guess.
Discussion
There is no perfect communication interface yet
usart, i2c, and spi are pretty decent, with the latter being quite fast for it's purpose.
Talking about that, smart cards use a half-duplex synchronous UART, except the fact that the frequency is ran through a fractional divider
Sometimes there’s parity. Sometimes not. Some versions of the protocol require you to pull the line low for a retransmit. Stopping the UART clock puts the card into sleep mode (usually.)
Another fun fact, contactless interface fully shuts down all systems including the NVM and CPU during transmit, with the only things being awake being the CL communications unit and RAM