What are the most common reasons a candidate gets rejected after taking online assessment? What shall we be mindful of?

It's important to remember that many of the simulations/online assessments are screened tests - think of the key words EY would expect you to be using, i.e. EY's core values, and how you might integrate this into your response.

You can also take numerical reasoning practice tests for free online - they pretty much mirror what you should expect in the interview process