For no BC, that simple case should be handled without requiring PropertyAccess

In a first time this is what I've done: check if element is an array, then that the key do not contains any "[]." (PropertyAccessor syntax) and the key exists in the element.

In that case directly access to the array value via the key.

And add an Else statement (not array and/or PropertyPath syntax, that contains the if statement (because the actual implementation accept that the key can be missing).

At the end, find simpler to just transform the key in PrpertyPath compliant syntax.

I'd configure factor for this method and onInvalid set null, otherwise it does not make any sense because PropertyAccess does not public constructor and only way to create Accessor is to call static method createPropertyAccessor

I'd do this:

if (is_object($element)) {
        if (!class_exists(PropertyAccess::class)) {
            throw new LogicException('Using UniqueValidator on object requires "PropertyAccess" component. Try running "composer require symfony/property-access".');
        }
 
        $this->accessor ??= PropertyAccess::createPropertyAccessor();
        if ($accessor->isReadable($element, $field)) {
             $output[$field] = $accessor->getValue($element, $field);
        }
}

I don't think validating deeper than first level should be allowed.
Otherwise we may end up with validating uniqueness of other object.