As far as I'm aware, cloudflare requires a domain name to tunnel traffic through to a server. Although they added the ability to connect to ut servers using DNS in v469, not everyone would use this so I would need to leave the server's IP directly accessible to the net which would still allow it to be attacked. Additionally, adding an extra hop to your connection to the server would definitely add at least 5 to 10ms or more to your ping.
Finally, it adds an extra level of complexity to things and even cloudflare sometimes has outages/disruptions which now affects a huge number of websites and apps when it happens. This is the main reason I haven't used cloudflare for this website even though its global caching ability is ideal in this case. I know you didn't ask about the website just wanted to give some extra insight to my reasoning here.
