I work in the private banking industry.
The key synergy would probably be around increasing the number of unique customers or markets/AUM between any merged entities.
I do think the type of industry dictates/drives the specific dimension as to how you would see synergy, but ultimately they would all be for the purpose of achieving a larger profitability/revenue than the sum of its parts.
To my understanding, assuming the usual objective is maximise profitability, I don’t think that metric differs by countries.
As to whether the synergies are realised, I think it is highly dependent on the PMI strategy, and whether the assumptions to achieve the synergies expected are realised/hold true.