Yeah, that is true with anything.. I hatched 8 out of 23 eggs with the hygrometer/thermometer that I've got.. 10 of the eggs didn't have any veins or anything in them, but I guess the thing can't be too far off since atleast some of the babies survived.
The problem that I ran into 3 days before hatch day was the power went out, I had to move the cooler-bator to a office building. In the process the bowl of water spilt out, when the babies started hatching I wasn't here. I lost two to drowning, there was one that piped on the wrong end and suffocated. There were two that were slightly less developed and died it the shells. I guess they died due to the temp flux during the outage.