I have a two Little Giants. One is still air with an egg turner used for incubation period. The other has circulation fan, this one is for lock down & hatching. Right now I am incubating in the still air with the turner in it. The temperature has been a steady 99F for the past 11 days. I have found them to both be very stable and reliable for a cheap incubator. If my hatches are off, I think the problem is most likely the egg not being fertile. Not the incubators. FYI, I don't touch the humidity while incubating, during lock down I'll bring it up to 55% to 60% humidity.
Don't be afraid to buy the cheap Styrofoams, whether it's a Little Giant or Hovabator.