Fortunately Graham is the only one I have come across who is so vitrolic in his distaste of trolley systems. You will find most of us will give you a far more balanced and objective viewpoint.
Ultimately they all do the same job don't they, you have a water container, be it a 1000l van mount or the 10 or 20l you may carry with a backpack.
You have a battery, hose, pump, pole and brush, and the windows are cleaned with pure water.
A van mount will have a big leisure battery that will power your system all day long.
A backpack has a much smaller one, if used all day long you will need more than one, and they also take well in excess of 24hours to fully recharge.
The same also applies to the Trolley system, but depending on the type of trolley, you can also use with large leisure battery.
A 1000l of water means you aren't going to run out of water, you won't have to keep coming back to the vehicle.
A trolley can carry up to 50l of water, you still have to call back to your vehicle for fresh water supplies, but 50l should enable you to do 3 houses.
With a backpack you are constrained by how much you can carry, if you are carrying 15l of water, plus battery and pump, then that all adds up. Ergo you will have to make frequent trips to top up your water.
Each method does what it says on the tin
It all depends on your own specific needs.
Ian