I guess it comes down to whether you want to pay Heroku or maintain the infrastructure yourself and pay your cloud provider. On the other hand, Deis Workflow had a lot of features that were similar to Heroku but Heroku Private Spaces seem more feature rich.

Closer mirroring to production environment and having one provider needing to be maintained.

That's only true if you are testing with Heroku but deploying to something else. But if you use Heroku for production you still only have one provider to maintain.

I assumed since they were asking they were using something else for prod. If you use Heroku for production, I’m not sure why you’d use something different for Dev.