tl;dr

The UniqueEntityValidator makes the wrong assumption that Doctrine's ClassMetaData::getIdentifierValues() can only return scalars:

implode(', ', $class->getIdentifierValues($entity))

Therefore, it crashes when the returned identifiers are objects (associated entities).

Details

When the DoctrineBridge UniqueEntityValidator encounters a non unique value, it builds a constraint violation with it. In case this value is not castable to a string it tries to build one itself:

if (is_object($invalidValue) && !method_exists($invalidValue, '__toString')) {
    $invalidValue = sprintf('Object of class "%s" identified by "%s"', get_class($entity), implode(', ', $class->getIdentifierValues($entity)));
}

If the entity's identifiers are associations (for example a Friendship entity with the two users as composite PK), $class->getIdentifierValues($entity) returns those entities. If they don't have any __toString() method either, the implode() function can't execute and it errors out with the following message:

Error: Object of class [...] could not be converted to string

Not sure what the complete value should be anyway. Recursing one level would produce a rather convoluted string...:

Object of class Acme\AppBundle\Entity\Friendship identified by object of class Acme\AppBundle\Entity\User identified by 123, object of class Acme\AppBundle\Entity\User identified by 456