Actually, a load balancer (beleive it or not) is a central point of failure in itself. It's also locked to a specific region unless you're Cloudflare and run your own DNS which many web users are currently using.
The advantage with
cf-api.99stack.com is that it's globally load balanced, if you don't mind your requests going through Cloudflare's servers then this is the best choice for you. The original endpoint
api.99stack.com is basically only within EU so it's fast and privacy friendly if you use it from within Europe, otherwise not so reliable and fast.
I think there's gonna be local endpoints too in the future, but keep in mind that the original
api.99stack.com will never be able to load balance and stay fault tolerant the same way
cf-api.99stack.com does. I've read somewhere that it's gonna be using round robin DNS load balancing which is pretty much like playing Russian roulette with your own application, just use
cf-api.99stack.com and if you want privacy in the future, make your own load balancing with the local endpoints.