I see this being a 20 page thread, like most other threads that cover this subject, but here is my personal opinion anyway.
I've run cheap spacers on my vehicles for years and have never had an issue. I see hundreds of Jeeps and Toyotas each year on the trails, many who run spacers. These same vehicles are most often daily drivers. I have seen two failures (that I can recall) since 2007 and both times was due to a failure to properly torque the spacer.
This is just my theory, but what I believe is happening to give spacers a bad name is that people do not re-torque them! When you install new wheels, especially aluminum wheels, you need to go back and retorque after a few miles. The same needs to happen with wheel spacers. The trouble is that wheel spacers are hard to get to so people neglect to do it. The lugs on the spacer loosen up after some time and then the wheel passes you on the highway. If instructions are followed, wheels and spacers are properly torqued and re-torqued, things should be fine. I use the word "should" because I can only speak from my experience. I know there are thousands of stories on the net of people who's wheel spacers have failed for absolutely no reason. Another thing that can happen is using the wrong lugs on the spacers. Many people have steel wheels and switch to aluminum. Sometimes people want to save money and use their steel wheel lug nuts on the aluminum spacers. This will not work well. The lugs will dig into the aluminum and even though there is very little clamping pressure you will reach your torque value on a wrench because of them digging in. This leads to loose spacers from day 1. This also leads to them loosening up much faster because they have very little surface area to hold onto. Use the proper lugs for the application.
Many Dual wheel trucks come from the factory with wheel spacers installed. If they can do it and offer a warranty and have to deal with a constant threat of baseless litigation, I don't see how the aftermarket can't do it as well.