I had a job offer from the StatusPage guys a few years ago that I turned down (shortly before their acquisition by Atlassian, d'oh!). The feature set is relatively simple, but the underlying software is very complex. They need to be reliable enough and robust enough to work when other major sites go down, which is no easy feat. I'm sure that like most products they started simple, but to scale to any large meaningful customers they had to put a lot of complex engineering in place.