I was wondering if are there some real advantages in using Buster on a Raspberry Pi Zero, or I should continue using Stretch
What are you doing with this Pi Zero ?
If you're doing some project without connection to internet or just locally (home automation, etc) I would say no. If no big bug endangering your project has been corrected it may no be worth the time and effort required to put again in place your project.
Now, if your Zero is hosting some web server, everything opened to the world it could benefit from kernel updates and other software updates. (Some of them could be patching vulnerability, etc).
In short it's down to safety en reliability against the time you will have to invest to update your project to this new OS.