I have seen digital thermometers read nearly 10 degrees off. Glass thermometers have their issues also. It may seem obvious but anyway, buy the best thermometers you can afford (within reason). We're hatching eggs not brewing nuclear fuel or something.
Glass/mercury thermometers can present a few problems, such as, they require you be relatively close to read them.
Mercury has been replaced with other materials in many of today's thermometers. But, for the sake of this discussion.... The following drawbacks to glass/mercury thermometers pretty much DO NOT APPLY to conditions found in an incubator.
1. Drastic changes in temperature, causing the mercury to travel up and down the tube can cause wear inside the stem.
2. Heavy vibrations from things like machinery can cause the mercury to separate.
3. Glass will break if dropped or treated too roughly.
Temperature is probably the most important variable to control in an incubator. An accurate thermometer is a must. I use a digital with a long probe that goes inside the incubator and, the meter with a large display for easy-n-quick viewing, rests outside. I have 3 glass thermometers that I move around to different places inside the incubator to make sure I don't have hot or cold spots. I bought all three glass thermometers at the same time, at the same place, and made sure all three were reading the exact same temperature.
If I had to bet the life of my chicken's hatching, I'd trust the glass over low-cost digitals every time.
