I have a farm innovators model it is a styro foam 42 egg model with turner, it has white plastic over the foam on the outside which I like for added durability. It seems to hold temp fine so far it will vary a degree or so from time to time however right now my room temp is fluctuating some as well and could be part of the reason. Only bad thing is the built in thermometer hygrometer aren't trustworthy but I would prefer to use a secondary thermometer on any incubator.
I think much of the problem with the temp fluctuation in these small incubators is lack of thermal mass, I am only doing a half batch of eggs so there is only half the intended thermal mass, I think it would be helpful to add thermal mass in the form of water in bags or even fake eggs in the open spots on the turner as this may help hold the temp more steady, my next batch will be a full load and I am going to try to add thermal mass in addition to the eggs.