Quite simply: at certain threshold counts of users you will be forced to add many more cloud instances/pods running Rails than you would need running node.js (or Java or go or many others). But it doesn't stop at instances because this will also require more persistent disk / object storage, more logs, more alerts, more notifications from the cloud provider that instance xyz needs to be restarted (due to a firmware upgrade or whatever), etc. etc. All of these have human management overhead costs and most of them increase monthly financial costs.