For some apps, where you can easily scale app servers horizontally and be stuck in a single redis (because redis cluster mode is not well supported by drivers,

For some apps, where you can easily scale app servers horizontally and be stuck in a single redis (because redis cluster mode is not well supported by drivers, or because it can't access some atomic LUA operations) you can get redis to be your number 1 bottleneck. Specially because you can throw more cores at the PostgreSQL so Redis quickly becomes the only thing that doesn't scale.