It's not hard once you unlock the bootloader. That's usually the most PITA part. Once you get past that stage it's no different in difficulty as installing Windows on a computer.
The only downside is if you need Safetynet. I used to use Lineage on a Xiaomi Mi 5 and it worked great 99% of the time. The 1% of the time when Google updated Safetynet and Magisk (or I) didn't update to get around it, it can be annoying, especially if you need to use banking or payment apps during that moment. The latter happened to me when I was trying to board the subway. XD