How cost-effective are fully custom WordPress builds for clients in the long run?

On the one hand, the benefits are pretty clear: better performance, much more flexibility in design and functionality, cleaner code, and less dependence on third-party plugins (maybe using only couple official like woocommerce and so on).

On the other hand, custom builds can make maintenance more complex and expensive. With a typical plugin-based site, you can often update plugins, test the main flows, and move on. With a more custom-coded setup, even a seemingly small update can require proper staging, regression testing, and QA to make sure nothing breaks.

That obviously increases the cost of ongoing maintenance for the client.
So where do you draw the line?

At what point do the performance, flexibility, and reduced plugin dependency justify the higher maintenance cost and complexity?

For those of you building custom WordPress sites for clients, how do you approach this trade-off, and what types of clients/projects do you think actually benefit from going fully custom?