Have a dual bladder -- one filled with air, and the other, becomes filled with water from the fresh water pump The bladder with the fresh water "pushes" against the container of the accumlator and against the air bladder. I beleive that in the system is usually downstream in the hoses/pipes/hot water heater/fixtures.

It's possible to have bladder leaks in the accumulator; however, in my years I've never seen that as an issue. I've had bad pressure switches, occasionally too low voltage (expecially with a Groco washdown kit, which draws a lot of power and usually is in the bow), and sometimes diaphram inside the pump (like a bad heart valve in your body).

For a new installation, I think that the variable frequency pumps are less expensive than the pump plus the accumulator. I'm not sure they are better in the long-run because of the power requirements and the cost of replacing more expensive pumps. But, I haven't had to deal with that, myself.