Can control surfaces for slow aircraft be 3D printed?
There are plenty of 3D printing methods available now with FDM being the most common. In my experience as small fixed wing UAV designer, all the 3D printing processes that are financially viable always under perform in comparison to alternatives. I can tell you the main issues I have encountered leaving the printing defects out:

additive manufacturing doesn't lend itself well to mass production.
It is too heavy for a core material.
melts under heat, Fire hazard and safety issues.
FDM prints are susceptible to fatigue especially when stress is transverse to the layers
for commercial use , the end customer doesn't like 3d printed parts on his aircraft. simple as that
machined metal parts are always stronger and cheaper than metal 3D printed parts.

However, it has some major benefits but mostly as a prototyping method. You can test the fits and physical size rather cheaply. I use it sometimes as a mold pattern for small parts. Also, 3D printing wings is not as common as you would think in RC world.
TLDR; with current technology, 3D printing is not suitable for stress critical components.