I'm afraid I don't have a single source – this is from discussions about tech teams, culture, engineering, etc, at meet-ups in London over the past ~7 years. They had a relatively large engineering team so there are plenty of ex-Hailo devs around here.

There were some good things to come out of it, from what I understand. I believe Hailo were avid Cassandra users and many of the team went to Monzo who have deployed Cassandra very effectively.