This is rather recommendation than issue. The code below sends a string message from UART1 and is supposed to pull the output pin LOW :

mypin.on()
uart1.write("a long sentence")
mypin.off()

when I checked the signal (part tx and output) with logic analyzer, I saw that pin is pulled low long before the uart transmission has been completed. A function like uart.flash() which checks the relevant uart status bits for completion of the transmission would be very handy.