[1] https://hackaday.com/2016/07/27/3d-printering-non-planar-lay... [2] https://github.com/makertum/non-planar-layer-fdm
Actually I wonder if internal waviness could be a useful method to add strength in general (area should be increased as ~1/cos(angle), or ~(1+angle/29) for very small angles, i.e. about 3% per degree).
The problem that results is the actual dynamics of the extrudate varies a lot, going through a fixed size nozzle: there's a relatively limited range of shapes that do not excessively impede flow (and risk jamming/clinging to nozzle/dragging) and are sufficiently smashed into the previous layer. So if you have layer thickness varying a bunch over an otherwise continuous layer, pretty easy to have poor adhesion in many places or pressure building and later overextrusion.
Also there's a fair bit of lag-- often hundreds of milliseconds- between extruder movement, pressure change, and extrudate geometry changes. When you're moving the nozzle at, say, 100mm/s over the surface, the uncertainty of this lag translates to a big uncertainty of where on the part extrusion thickness changes.