First off, WordPress developers specialize in programming.
Real developers are platform-agnostic and often language-agnostic since the concepts of software development revolve around solving business problems through algorithms, using data structures and paradigms embedded in the OS layer, web servers, the networking infrastructure and more.
WordPress developers are software (or web) developers who are familiar with the WordPress Core codebase and its APIs. They are in charge of building applications on top of the WordPress platform.
That may require theme development, framework design, creating plugins and extensions from scratch, scaling WordPress multisite networks, troubleshooting performance, security, stability issues.
Building a WordPress project requires a theme as well – serving the presentation layer of the website. Theme developers are those who profile in building themes. Site builders/customizing folks are those who may tinker with some CSS and HTML here and there and know a couple PHP actions or very basic jQuery.
The theme itself should be kept simple and to the point. Features are built as WordPress plugins in order to avoid a vendor lock. This is common with premium theme users. Whenever a theme contains new features, switching to a new theme would prevent the site owner from leveraging those as they are an integral part of the previous theme.
Themes or Frameworks WordPress Developers Specialize In
For theme developers – or capable WordPress programmers who are also in charge of the presentation layer – specializing in a certain framework may be an “okay” option.
Underscores is a good starter theme that requires CSS but provides a standardized markup that works in most cases.
Genesis is not a framework by definition as you can’t integrate it into an existing product (theme). It’s a parent theme that happens to have a large collection of child themes that are added on top of the core product. It adds on top of what’s available in the parent theme.
Genesis is generally stable, compliant, and fast. I’m not a fan of its architecture as it requires you to remove areas instead of building atop a foundation. It’s popular and works well in certain cases – and some people have specialized in building on top of it.
If you’re looking for a starter theme that’s overall flexible and stable, Genesis may do the work. However, I’d suggest starter themes like Underscores for theme developers and using the right titles for the different roles (or people) working in the WordPress ecosystem.