Whiteboard interviews look at a small sliver of skills you'll need for a software engineering job namely problem solving and mathematical thinking. Other equally important skills are communication with colleagues and non-technical team members, design & modeling, familiarity with domain knowledge (especially in niche companies) & perseverance (a good developer is aware of the inherent frustrations of dealing with multiple layers of abstraction). A better way to test candidates would be to ask them to add a small feature/resolve an issue on your existing code base and pair them with an experienced developer (with due compensation for time). But this is not a scalable process as it calls for equal investment from the companies' side as well.
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