What would happen if the baud rate between the two devices are not synchronised. Let’s say on the second device there is a rising edge on its clock after some short time after the first device, so the first rising signal is on the first device. Even if the second device starts sampling 16 times it’s baud rate, it has to count a low signal, but because its clock is off, its sample to know that data will be sent will overlap the first byte?
To my understanding the recieving device starts to check the data when it recieves a zero for 16 times its baud rate, so if the transmitter has lower baud rate, the transmitted zero would be too short making the reciever thinking it's a glitch and not checking the data. If the baud rate where higher on the transmitter, then the reciever would read the bits wrong, that could result in making the reciever skipping bits, getting wrong input and so on.