Be pragmatic: "ACF for client projects where speed matters and the plugin dependency is acceptable. Custom meta boxes for plugins or themes I distribute, because I cannot require ACF as a dependency."
ACF (Advanced Custom Fields): fastest development time, great UI, field groups with conditional logic, repeaters, flexible content. Downside: plugin dependency and potential vendor lock-in. Custom meta boxes: native WordPress, no dependencies, full control, but more code for complex field types. Built-in Custom Fields screen: too basic for anything beyond simple key-value pairs. The choice depends on: project budget (ACF is faster), long-term maintenance (native is more durable), and field complexity (ACF repeaters are hard to replicate natively). Strong candidates discuss: storing ACF data in postmeta and the serialisation gotcha, the register_meta() function for REST API exposure, and Carbon Fields or CMB2 as alternatives.
Tests practical WordPress experience. Candidates who cannot work without ACF may lack understanding of the underlying meta system. Those who refuse to use ACF on principle are ignoring a tool that saves significant development time. Look for nuanced judgement.