For me it's not a big deal, my app autoscales and rate limit details are stored in Redis, so it's super fast. For x100 the traffic it wouldn't even need to scale since it's simply hitting Redis.
Interesting, but still your Rail app needs to apply the rate limit logic working with Redis, maybe your DB will be protected, but you will still receive the traffic through your network, meaning more ingress cost, along with CPU to process all the incoming requests.
Ingress is free with most providers. Yes, of course it adds a bit pressure on the cpu, but yours would add latency, might add peering issues, additional costs, etc. and I have no control.