They're owned by their members (hence the name), and operate pretty much exclusively as an old-school retail bank without any of the other banking activity that tends to at least wander into ethical grey areas.
I used to think that Mutuals would have their members interests first and foremeost. Certainly they never tire of telling people that. But my actual experience is that it's not true. Mutuals, commonly, are like any business where the owner (the membership) isn't paying attention and the result is a business that puts the interests of the staff first.
Service to members of a mutual society can be every bit as bad as any public listed company and is often worse because of the lack of a profit incentive means no one gives a damn
That's a bit of a strawman I think — surely by the logic you're using, the existence of a profit motive would mean that the company was interested only in serving its shareholders and would ignore its consumers? There are plenty of examples of this, but it doesn't mean it's true generally of profit-making companies.