ETTL is not consistent, simply because the scene content can vary slightly or radically from shot to shot, such as due to subtle change of lens FL, not because of inherent inconsistency of the flash unit. Inconsistent, but quick to respond to changing circumstance (such as distance to subject from light)
Manual flash is consistent because it simply outputs a predetermined quantity of light for each shot, and it cares not if something bright moved into the scene or that you changed subtley the FL of your lens. But if the subject moves you do not have a adjustment of exposure to suit the new light-to-subject distance, so it is less nimble for dynamic shooting situations!
Separate issue from ETTL/Manual flash, is the question of use of Av/Tv vs. M on the camera! My suggestion is M on camera, ETTL on the flash, with use of FEC.
