Concept of WP-United is great. It also worked ALMOST well out of the box, but needed some customizing both PHPbb and WordPress side before I could go live with it. I also used separate templating on phpBB and WP to avoid problems.
After using WP-United for over 6 months on our live phpBB3 & WP website I can't recommend WP-United to anyone for live usage. The plugin is actually ABANDONED as there is no response on developer website or at support forums.
If you're not good yourself in modifying and updating phpBB3 and WP codewise, it's just too risky to use this plugin.