I've just had a nightmare cleaning up an unintentional merge. Your merge tool requires (potentially) several actions to merge one list from another.
Your previous tool was fairly self-explanatory. You clicked the lists on the left hand side, and merged them to the lists on the right hand side. Then you could go and delete the lists.
This new tool shows you where all (check), some (dash) or none (blank) of the email address to be merged already exist. My natural inclination was to uncheck everything except where I wanted the email addresses to merge to. This action removes those email addresses from the lists they're already in, or partially in, without warning or explanation. I was left with several blank email lists and no assistance to recover them.
We were finally (after :45 minutes on the line with tech support) able to resurrect the old lists. This in my humble opinion needs some more explaining on the user tool... AND some un-do options.