On Pixel devices, the UART is not configured or brought up by default in locked production mode (as things should be), but by unlocking the device and then using `fastboot oem uart enable` you can flip the bits to turn it on. On early Pixel devices it was on the headphone jack and on newer ones it's on the SBU pins.
By default I think it's still configured as the kernel console in the kernel command line, so once it's enabled it will show the kernel debug output and present a TTY. But of course you can subsequently configure it to do whatever you'd want a UART for: kgdb for kernel-debugging, earlier stuff in the bootloader, and so on.
So, the implications are just: there's a convenient debugging interface available to you that turns on if you unlock the device and ask for it.
On Chromebook devices there's a more complicated and fancy debugging system where the SBU pins can be muxed to the security processor's USB host interface by presenting a debug cable called a SuzyQ, which presents a whole suite of debugging facilities. This used to be used quite frequently for unbricking purposes.