I didn’t get my WFP system until I started to do multi-story commercial buildings. However, I was able to offset the cost of the system and pole by doing a lot of pressure washing with the equipment I already had. One visit covered the expenditure of the WFP tools as well as the overhead for that single job, but that was because I already had the PW equipment to do a large building.
That being said, I find myself using the WFP on all of my residential maintenance cleanings since I usually do a traditional tool cleaning for the initial visit and then keep my clients on a 6 month rotation. 95% of my business is residential. I prefer to use the WFP when I can to save my shoulder and elbow from repetitive movement from fanning with the trad tools, but it also helps extend the life of my strip-washer/mop/sleeve. Since the exterior is often the side that is going to have larger dirt particulates, I use the WFP with a hybrid brush to do the cleaning in order to preserve the fibers of my microfiber mop. Basically, I scrub the exterior and the interior is more of a dusting.
I do find that using the WFP is about 1/3 faster, but that can vary depending on your equipment set up time. My set up is highly customized and is more of an exception to the norm rather than an example. Where I do see it saving everyone is in the safety factor. It helps if you are doing multiple multi-level buildings by keeping you from needing to use and move a ladder around all the time, especially on uneven ground.
As far as the windows coming cleaner vs traditional, both have their positives & risks. Traditional means you must be within arms length of the glass unless you are really good with a pole (and believe me that in and of itself takes a lot of practice), but the glass typically ends up clear and dry. The risks are needing ladders, chasing streaks from too much soap on the exterior, chasing water along the edges, etc. These are more of time sucks instead of physical “risks”.
The benefit to using a WFP is that you can scrub and rinse and move on very efficiently without having to leave the ground most of the time. However, you have to be on top of your TDS as well as know the state of the window frame and paint as Luke mentioned. Too high TDS and you will end up with water spots. Loose frame or trim parts around the window can trap water and coalesce your pure water with dirt particles and then release streaks hours after you have left the job site. Using a brush around oxidized paint just ends up smearing it all over the glass resulting in another time suck of multiple rinsing passes.
Lastly, consider what your main client is. Residential or commercial. If you are doing a lot of commercial routes with strip malls and restaurants, using a WFP for everything is going to be much more of a hassle since you will need to source a water supply (which may or may not exist, or be locked behind a commercial plate) and set up your WFP system and run hoses which exposes you to a tripping hazard and potential liability. These are huge time sucks compared to traditional tool cleanings. Conversely, using a WFP at a business location that is closed/during off hours so that there are no patrons to be concerned with can be easier, especially if it is a multi-story building, will save you time and energy.
If you are doing a significant amount of residential work that is single story I would hold off on purchasing a WFP system. Build up your speed traditional cleaning and save your funds to get the right set up for when you are ready to expand into larger projects.
These are my recommendations from my personal experience. Do with it what you wish.