The competitor is BYD and Toyota, and an overall shrinking car market.
Mergers are often fraught with evil so who knows what happens. But if there going to compete with BYD and the other Chinese companies they need to scale up EV drivetrains. EV drivetrains are a pure commodity. You can't have 20 different companies producing a commodity - the big players will thrash them, I can rattle off 20 industries where that has happened, and it doesn't take long.
EV's are an appliance.
Also, of all the Japanese companies Nissan and Honda don't overlap as much as most think - both in vehicle types and also countries where they are strong.
Nissan's trucks are very well regarded around the world. I worked with a guy who owned a large pistachio plantation in South Africa and the only trucks he would consider for it were Nissan. That's not even including the UD trucks - which apparently are no longer Nissan so I guess that doesn't count.