It's fine for simple apps, but it's not suitable for scenarios requiring complete container orchestration, such as service discovery across numerous containers, automatic scaling, and coordinated application upgrades. Also, I believe it is not stable enough for production use; it is better suited for testing.
And the Azure App Service costs are determined by the service plan, which is billed per second. The App Service provides more functionality than Container Instance, and you can plan your use according to your needs. If you're not happy with App Service, you might want to look into the Azure Kubernetes Service, which offers more control and features than the Container Instance.