Our agency builds wordpress websites using 100% custom built wordpress themes based on underscores but using Bootstrap 4 as our framework. Bootstrap offers the flexibility we need to quickly build bespoke designs.

We used to build our sites using ACF flex content blocks, but now are using ACF's Gutenberg blocks.

Because of the way we prefer to build our sites in modular sections, we cant allow our clients to use any core blocks except those we explicitly define to work in gutenblocks we have setup that use innerblocks. Otherwise they wouldnt be contained in bootstraps container>row>grid markup and would break the layout of the website.

As an agency we need absolute and total control over the markup of our websites. Gutenberg is a threat to that total control and would allow clients to build layouts that are outside of the brand standards.

I think there is a huge division between regular people who build janky websites and would be totally fine with getting an off the shelf theme, and just adding whatever blocks they need - vs Agencies and companies who need total control over the design.