@GiuseppeArcuti can you, now that there is a reproducer on the issue?

What should be done next here? Anyone up to help finish this PR?

What should be done next here? Anyone up to help finish this PR?

Either it should support symfony uid properly, or add an extension point to add custom equality checkers or some sort of callback option?

No one tagged me.
@GiuseppeArcuti , thanks for creating the test.
There is, of course, an issue - I mistakenly assumed that the identifiers were scalars.
If you're still working on this bug, replace line 200 in the UniqueEntityValidator class with the code below:

                if ($fieldValues[$identifierFieldName] instanceof \Stringable) {
                    $fieldValues[$identifierFieldName] = (string) $fieldValues[$identifierFieldName];
                }
                if ($propertyValue instanceof \Stringable) {
                    $propertyValue = (string) $propertyValue;
                }
                if ($fieldValues[$identifierFieldName] !== $propertyValue) {

This should resolve the issue in most cases.
There’s still a separate problem with object-type identifiers that may not have a toString method. However, that’s part of a different feature (another PR) that will introduce something like a normalizer.
I’ll take care of that in a new PR.

Anyone up to help finish this PR?

@nicolas-grekas If the PR has been abandoned, what’s the procedure in that case?

You can just open a new PR and reference this one and the issue, thanks

Continued in #60275, thanks for the PR @GiuseppeArcuti