While rooting itself is safe, and wouldn't cause issue by itself, please avoid the following things otherwise you risk bricking your TV.
DON'T: Modify System Partitions
DO NOT write kernel, rootfs or any other read-only partitions under any circumstance!
webOS bootloader will verify the signature of those partitions, and refuse to boot.
DON'T: Mark files non-writable
chattr command can be very dangerous on webOS. Some script will fail to run (e.g. start-devmode)
if files used in it can't be modified or deleted. You may lose shell access to your TV forever, even if you
factory reset your TV, as user partitions will get its files
rm -rfed rather than formatted.
What if I bricked my TV?
We've warned you. But you're welcome to ask for help on the OpenLGTV Discord server. It's still possible to rescue your TV, but opening up the TV and reprogramming the storage chip is what needs to happen most likely. Or perhaps you'll require a mainboard swap.