I'd argue that's still not a very good comparison, because for ROMs you have to go through the trouble of researching the specific method for your phone brand, model, & firmware version, learning adb, unlocking the bootloader, flashing a custom recovery, then from there you can install a custom ROM. Then if you fuck up you have to figure out how to debrick the device.
While Linux(user friendly distros specifically) is just burning an ISO to a USB, possibly changing 1 or 2 settings in the BIOS & booting from that USB, then just clicking through a graphical installer like calamares.
The barrier to entry is drastically smaller.