"With all that, the last 20% of a Cordova app may well take 80% of development time, and that's rarely been budgeted in."

To be fair, that could be said about most software projects. "80% done, now the last 80% remains."

Not that I disagree with you, I had the same experience trying to make an Android app in Phonegap. It's lacking, like most cross-platform frameworks (for most types of apps, but typically not games) I have tried. They are only suitable for basic stuff where the native feel isn't critical.