The spell is not fine, not at all. Barkskin needs a rewrite, desperately. In fact, I think it's the most poorly written spell in the book.
Just looking at the RAW of the spell and looking at a Druid wild shaped into a bear with AC 11, I think that the spell is not all that bad. It's a pretty kick butt spell in that scenario.
The problem comes in when people want to use it mechanically like other types of armor and/or other types of AC spells.