There are few things to consider to choose between custom developed Theme and integrating with existing Theme framework.
Time & Cost
There will be no change in delivery time and cost for either options unless you need extensive features from the Theme framework. For example, some real estate Themes have features like advanced search which is beyond core WordPress features. For developing custom theme from scratch, additional features will incur additional cost but there will be no change in delivery time. We deliver custom developed Themes within 24 hours.
Most premium themes are bloated with many features for broader market base. You may not use most of the features but they will affect the site performance. Custom Themes will be leaner and faster because they are developed to suit client’s requirement.
Updates & support
For sites built on existing Theme frameworks, clients should rely on Theme framework authors for future updates. Their updates may conflict with the child theme. You will incur additional cost to ensure each update does not break the site.
We recommend our clients to get the Themes developed from scratch for efficient code, better performance and reliable support.