Every time I used IPv6 I found it solved more problems than it created. E.g. with v6 you can make sure a VPN addresses will not collide with the user's actual address. No more nats or port forwadings, etc.
The main problem is that v4 has not yet been retired and that means many times you have to support both.
The main problem is that v4 has not yet been retired and that means many times you have to support both.