all 4 of my tanks are acrylic. 2 have stick on thermometers AND in tank thermometers, the other 2 just have in tank thermometers.
The stick on and in tank differ by 1° in one, and 2° in the other mot days, but once in a great while they will very by 3 or 4°
I actually think the one is not inaccurate at all it is on the left side of the tank, while the in tank thermometer is to the right of the center, and the heater is on the right. So being 1° lower makes sense.
The other one I know is completely wrong as IT is just 2'' away from the heater, while the in tank thermometer is on the other side of the tank and still reads 2° more than the stick on... which makes no sense since it's further from the heater. Those are the same company too, so you MIGHT be lucky and get something accurate, or you might get nonsense that can potentially cause harm to the fish via inaccurate readings :/
Good to know about the glass vs acrylic heat conducting tip. The packages don't mention it, but they do say that if you unstick and move it that you may damage it so that it wont work properly anymore.