Rails servers like Unicorn can still only support ~4 concurrent requests on regular dynos (roughly 1 per virtualized core), lessening the problem but not solving it if you have many requests per second.
The problem with only 1 concurrent request is that if that request takes long to process then it can block other requests that are queued up in that dyno. Lets say that 1% of the requests take long. If a dyno can process only 1 request concurrently then at each request there is a 0.01 chance that that will block other requests. If however it can process 4 concurrent requests, then the dyno will only be blocked if it gets 4 long requests at the same time. So there is only a 0.01^4 = 0.00000001 chance that a dyno will be blocked.