Risk of Installing a Custom ROM, with no more future updates
Well using a custom ROM which won't get updates means you will have to live with it's bugs (if there is any) or fix it yourself if you know how.
In my opinion biggest 'risk' when you use custom ROM is voiding warranty because you need to unlock bootloader. I don't worry about security updates because I'm carefull what .apk I install and on what link I click, also I don't think I'm interesting to attackers, and I'm sure you aren't too. If you have super sensitive information than you should use phone which get updates regularly, and do not root it or use unlock bootloader. But however even than if you not look on what you click you can let someone in. So don't worry about securty updates if you use your brain.