Providing you are committed and prepared to put in the work with your ops manual and concentrate fully through the 2/3 days then you will be fine.
However, it is no walk in the park and it is a lot of information to absorb in a short space of time.
Some find the flight assesement the most daunting, others find the idea of writing a 50 page operations manual scary while for some the groundschool exam is terrifying! (Pass mark of around 75%).
Your NQE will help you all the way through and they are there to pass people and get them PFAW not fail people.
Should you fail your Groundschool exam you are usually allowed one free resit (within a specific timeframe). Depending on the NQE, most will not allow a free resit of the flight assessment (after all you are expected to be able to mission plan, do risk assessments, site surveys and actually fly the thing) so you will have to pay if you fail that.
Your ops manual will not be submitted to the CAA until your NQE is happy and signed off on it.
Hope that helps.