Upvote:4
As RedBaron says, all the message means is that they are not able to make a decision in the time allowed. You should be "worried" in the sense that you will not receive a decision in the Priority Service timeframe. This may impact your travel plans. It does mean your case is not straightforward, but it does not mean your application is going to be rejected.
"Needing more information" does not mean they need more information from you. You have been asked for nothing, so send nothing. They explicitly say they will ask if they need more information.
Do nothing until asked, but be prepared for a delay.