WordPress Posts are not stored in a single table. Postmeta is stored elsewhere, and of course each table has its own format. If you ever publish content directly in WordPress (posts, pages, attachments, etc.) you could quickly run into issues with post IDs. Even if not, it would potentially require some fairly significant data manipulation to convert your non-WP data to WP-useable structure.
In this case, I’d say a REST API would be a much better way to go, and it would also be more forward-compatible.