1. Highly domain-specific legacy pricing model running on... Excel spreadsheets. These present huge performance bottle-necks and aren't easy to migrate.
2. Reliance on 3rd Party data. Did a scout just send incorrect data? Did their device disconnect? Did they rollback the wrong information? Is the source lagging behind the TV/online feed? GGs. Suspend the game and refund bets.
The tooling really isn't that important. Scalable software can be written in any software, and while some languages do make things simpler, often you can just also find a suitable paradigm (or middleware) that can provide similar advantages. Having said that, we do make extensive use of RabbitMQ (which is written in Erlang) in our environment. I suppose this is also just an example of my previous point though...