Any particular reason the UART0 is blocked for user code?
What are the implication of using it? What are the fun errors?
This can drive the hardware design choices.
Thank you

STATIC mp_obj_t machine_uart_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
    mp_arg_check_num(n_args, n_kw, 1, MP_OBJ_FUN_ARGS_MAX, true);

    // get uart id
    mp_int_t uart_num = mp_obj_get_int(args[0]);
    if (uart_num < 0 || uart_num >= UART_NUM_MAX) {
        mp_raise_msg_varg(&mp_type_ValueError, MP_ERROR_TEXT("UART(%d) does not exist"), uart_num);
    }

    // Attempts to use UART0 from Python has resulted in all sorts of fun errors.
    // FIXME: UART0 is disabled for now.
    if (uart_num == UART_NUM_0) {
        mp_raise_msg_varg(&mp_type_ValueError, MP_ERROR_TEXT("UART(%d) is disabled (dedicated to REPL)"), uart_num);
    }