FriendlyPerson The site clearly demonstrate this is a feature of the platform but is it really "set it and forget about it"
This depends on how resilient you want your system to be.
Let me elaborate, nothing is truly "set and forget" if the goal is 100% up-time. But it's the theory. We offer a 99.99% up-time guarantee: https://www.99stack.com/legal/service-level-agreement. Now all servers runs on the KVM hypervisor with the virtual disks stored on specific storage servers configured with raid.
In case something goes wrong with the physical host node, running servers will be live migrated to a healthier host node within the data center. This usually takes seconds but could take a few minutes in worst case scenario.
So to answer your questions, you don't need to do anything in case of a host node issue, the on site staff will take care of it and make sure your server stays up with smallest possible downtime. But as for 100% up-time, yes you do need at least two servers, preferably in different data centers to be fully guarded against any downtime.