The vertical accuracy depends entirely on your project GSD, individual image quality and quality of the entire dataset.
The lower the GSD (ie the images have a high zoomfactor or the flying altitude is low) the more accurate the vertical accuracy is in theory.
Generally speaking the proposed accuracy is 1-2 times the GSD for the X and Y and 2-3 times the GSD for Z.
For instance if you're flying a project with a native GSD of 5 cm, your expected Z accuracy would be between 10 and 15 cm in ideal conditions (ie sharp and well lit images, high overlap (80/60% for instance))
Obtaining an aboslute vertical accuracy less than 3 cm with RTK GPS is achievable with enough GCP's which need to be well distributed in the project area.