Great question, There are quite a few differences, namely in our product design focus. We've taken a more configurable and flexible approach that focuses on plugging into a companies existing stack and their process. Often times we'll have customers send us their entire playbook on what they have now and ask us to automate that as a starting point (e.g. rename Slack channels to my Jira number for incidents, etc). We do this to hopefully reduce the amount of change required when a new tool is brought in. As a result we focus on features such as our Workflows engine that allows for this customization. Another big area of focus for us is unsurprisingly Slack, we think of the other areas of Rootly such as our Web platform to be the backend that powers this.
FH does a lot of things well and has great customers too. They have a sleek UI, strong security posture, and more. Their approach is more opinionated in guiding you through incident best practices. There is no wrong answer here as we hear plenty from customers that want both.