We at Air think our features are most helpful when used in conjunction. Your Boards are used for basic organization structure, while Keywords & Tags are used to describe the content-based properties of the asset. Custom Fields allow us to sort and assign when actions need to happen.
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Think of it this way:

Keywords: a robot with no soul scanned your static images and found some nouns and verbs and used them to help you answer the question - What am I looking at?

Tags: a human with a soul looked at a file/image/video and used tags to help you answer the question - What am I looking at?

Custom Fields: a human used these in partnership with tags and keywords to answer the question - What do I do with what I'm looking at?