Dead reckoning is also inaccurate due to unknown winds. Even if you take off with the best available forecast, it’s often wrong by 5mph+. After three hours your position is off by 15 miles. That’s not remotely good enough for most aviation purposes.
Dead reckoning based on heading and time alone, but if you have a good INS it's not affected by wind only by accuracy of the gyroscopes/sensors.
Current systems drift by about 0.5 miles per hour. And that's normal commercial grade systems, I'm sure the military has an option for better systems if they need them.
As t0mas88 said, you don't rely on the measured speed of the aircraft/ship, you rely on gyroscopes (which are much more resistant to external factors), and both clocks and gyroscopes have improved signifcantly (clocks are now better than cesium atoms - read about atomic clocks) and so are gyroscopes (again, optical gyroscopes that are miles better than mechanical ones).
