Forand related issues we decided that we'll only support automated downloads for libraries for which the vendor provides a "latest stable" release download URL; e.g.
In essence, Libraries API behaves exactly in the way it was designed for: Determine the actual version, decide whether that's supported, and if so proceed with loading/integration.
However, I could easily foresee cases in which a library's latest stable is compatible with, say, D7 only, but not D6. Easiest examples are probably PHP libraries; e.g., the latest stable might have been revamped/rewritten for PHP 5.3 (D8 only), and D7 and below need to grab an earlier version - or alternative latest stable download URL for PHP <5.3.
Thoughts, feedback, drinks?