Now you have to pay for 2 services instead of one (elasticache is not cheap), and you may need to account for two differently configured redis instances (setting it as cache store requires a different configuration than setting it as a job queue).

You'll also need to code for two integrations (orm + whatever you're using redis with), which may be a solved problem or not, depending of your stack. And even then, still more complex than just postgres, and more error prone considering you'll either have to ignore enqueue reliability, or find a complex way around it.