The oil pressure gauge isn't known for its accuracy either.
The biggest indicator of how inaccurate the coolant temp gauge is the fact that the needle barely moves in the meat of operation temps (where you'd want accuracy the most). While the car is in motion, for the most part, if the system is full of coolant, the engine shouldn't change much in temps unless you're beating on it. In traffic, around town, stopping at stop lights, the fan kicks on and off every few minutes. That's probably a good 20-30 degree swing, if not more in temps and I can say that my needle doesn't move at all between the fan turning on and it turning back off again. With no coolant, the stock gauge will peg high for sure once the engine gets hot, but at that point it's already way too hot for comfort.
I have an aftermarket oil pressure gauge and it moves A LOT (compared to stock). It constantly changes with throttle input. It also changes with wide variations in engine temps (like start-up/warmup).
The fuel gauge and battery volts are pretty accurate for the most part, and the stock speedometer is as accurate as your tire size and tuning allows it to be.