Hi there :)
when solving cases, I often find it a bit unrealistic to draw an accurate MECE issue tree at the beginning. Therefore, I have started dividing my analysis in two or more steps. For example: if I have a market entry case, I would do it as follows:
(1) draw issue tree to assess whether the market is interesting (size, growth, profitability, barriers to entry etc.). I would communicate that I would think of a specific strategy how to enter if we deem the market interesting. If not, I would think of alternatives to reach the company's goals.
(2) based on my analysis of the market, I would come up with an issue tree for either the market entry strategy or for possible alternatives.
I think that such an approach is far more accurate as the initial assessment of the market would allow me to come up with a more precise structure for the following issue tree(s). Would that be an approach that would be acceptable during an MBB interview? Thanks a lot :)