1. It goes quick.
Once the paint hits the water, you're committed. That's why it's so important to have all the prep work finished and tools handy before you start. Even if you're doing only a section of surface at a time, you can't turn your back on it until the water stops flowing.
2. It eats canvas.
Complex pieces take days, but a free-form WDP session can consume any size surface in a matter of seconds. I've only tested it on 48x48 as the largest surface, but I'm not foolin' -- seconds. And then I spend minutes trying to control the water flow.
3. It's too much fun.
I just counted and I've produced 31 pieces since June 16, not including the 8x10s, sketches, and working/test surfaces like the walls and tables, all covered with paper drops cloths. It's habit-forming. Must be the thinner and oil paint fumes.
4. It's messy.
Thus a major part of the creative appeal! Tool clean-up is vital, and I covered the floors and walls so spillage won't damage the apartment. Splatterwork of any sort can throw paint a dozen feet or more, so be warned. However much coverage you think you need, triple it. Or work outside or in a barn or warehouse.
PS: You can't roll canvas with WDP on it. It wrinkles the paint.