Great question, and one that can be reviewed in significant detail.
High-level - mergers and acquisitions (M&A) are conducted to improve the company's financial performance and return for shareholders. A synergy is the interaction or cooperation of two or more organizations to produce a combined effect greater than the sum of their separate effects. If an M&A is conducted to create greater efficiency/scale, then the result can be referred to as synergetic.
For the CBA and NIC Bank merger, this combined the market share of the bank to 9.9% and expanded their customer base to over 40 million individuals in four East African countries (source: https://weetracker.com/2019/09/27/cbk-announces-nic-cba-merger-unveils-name-of-merged-entity/). Because the customer base increased and assets increased as well, this could be referred to as a synergetic merger.
Depending on the discussion, the opposite could be said about the "takeover" of NBK by KCB - KCB acquired 100% stake in NCB through a share swap deal, comprising of 10 ordinary shares of NBK for every 1 ordinary share of KCB. The question here becomes was this share swap advantageous for NBK's shareholders? It depends on the respective companies' share prices at the time of the acquisition.
Hope this helps! Happy to discuss further if you'd like!