When the ValidatorFilter creates a ValidationException, the exception message is left blank. That means that when the message is sent to a logger in the StepAggregator $this->logger->error($e->getMessage());, it will send an empty message. That's not very helpful when you're trying to log why an item cannot be processed.

My suggestion would be to set the message to a string that contains the line number and a comma separated list of all the constraint violations. That way the exception message makes sense, and of course you can still process the messages later.