If there's no particular reason to have unsigned userdebug builds, it would be more secure to have signed user builds instead. LineageOS ships userdebug builds but tweaks them to make them much closer to user builds (with settings to enable features that are only available for userdebug, such as root access). Concerning signing, it would be prudent to do so to avoid easy malware installation.
Additionally, there should be some reasonable performance uplift from the serial console being disabled.
If there's no particular reason to have unsigned userdebug builds, it would be more secure to have signed user builds instead. LineageOS ships userdebug builds but tweaks them to make them much closer to user builds (with settings to enable features that are only available for userdebug, such as root access). Concerning signing, it would be prudent to do so to avoid easy malware installation. Additionally, there should be some reasonable performance uplift from the serial console being disabled.