Oh Yeah, They can cause some serious damage on some surfaces, especially if one is using the wrong nozzle head for the type of job they are doing. I use the small one I have for the tile underneath our place but I am very careful not to get it to close as it will pit the tile. I usually use it to just like loosen up the dirt them squeegee it off.
I got a laugh out of your "one section that is done and the other that it isn't"...
. Once you commit to an area you have to finish it. If not, it looks terrible.