I was always under the impression that integrating unknown biases in gyroscope and accelerometer measurments is the main issue with dead reckoning.

Today I am convinced that attitude errors are the far bigger issue, since they will make you misinterpret gravity as genuine acceleration.

Even with a 1° error in your pitch estimate, you will start integrating 9.81ms^-2*cos(1°) = 0.17ms^-2 as a forward acceleration. Within one minute, you are 308m off already. This is the real issue with dead reckoning IMO.