I have occasionally used the illegal taxis myself when desperate, but these were not included in the general primer because I don't think they are a good choice for someone not familiar with the territory.
1) You need Mandarin skills.
2) You need to know where you are going...as in the city and the route.
3) You need to be desperate and have no other options.
While my ventures into this area had no detrimental outcome other than my wallet, on one occasion after about 100 meters, driver stopped and a second man (obviously someone he knew) got into the car and I raised holy h*ll and jumped out immediately. It was a set-up for a robbery...or worse. It felt all wrong. This guy had a fake "meter" and medallion on top of the car. But as I only noticed once I was out of the car--both his front and rear license plates were completely covered up. Bad news.