Redis starts to have issues at high scale, even on sophisticated hardware, that can be quite difficult to debug without a lot of additional effort and storage. It’s not just memory, but odd behavior (e.g. randomly dropped connections) with a lot of connected clients, or hot keys/nodes in a cluster configuration, etc.

These issues can exist in any system, but in my experience it’s especially tough (relatively) to identify and diagnose them with Redis. Once you add lua script usage it can get even worse.

A traditional RDBMs or filesystem is designed for high throughput and concurrency, even if some tasks are blocked on data. Additionally both have options to partition steadily growing things. If needed with old partitions being moved to tape backup while the server continues running.

Redis is a single threaded program acting against RAM whose philosophy is that it does things fast then moves to the next job. If it needs to access memory that got paged to disk, the whole server stops and waits to get it. Nobody can do anything.

Because Redis doesn't have to deal with locking and concurrency, it can run much faster on the same resources. But when concurrency is required, it is stuck because it doesn't have it.