I don't think the styro incubators are a problem. I just had a 100% hatch on two breeds and an 80% on another. I think the biggest problem is the wafer thermostats, styro or not. They just don't keep a super accurate temp and are prone to spikes. If you read through the posts on here about temps going up to 103, down to 95, adjusting it up, adjusting it down etc... I would bet every one of them was regarding an incubator with that type of thermostat. I use a forced air hovabator with digital thermostat and would not trade it for anything. When I had an older model with wafer I had those same problems. I was trying to hatch eggs I paid anywhere from $35 to $85 a dozen for and had miserable results. One of the main problems is that minor temperature fluctuations are not necesarily fatal, but humidity needs to be adjusted to compensate for them. It is nearly impossible to adjust things quickly and accurately enough in that type of incubator. Of course, if you are looking to hatch more than 42 eggs at a time you will need a bigger machine.
Richard