Pressure altimeters have 3 main errors:
1. No machine is perfect, so instrument error, lag and hysterisis will affect any such altimeter to a degree.
2. Installing an altimeter in an aircraft will induce certain errors, such as position error and cockpit temperature.
3. Even if both altimeter and installation are 100% perfect (impossible), the atmosphere in which the aircraft is flying probably isn't 100% International Standard Atmosphere, to which the altimeter is calibrated. The resulting error is termed 'barometric'.
These errors may be additive or subtractive - but they will always be there.
A good pressure altimeter is probably accurate to ±30 ft - but that also assumes that the pressure setting datum is accurate.