As Sequoia said : no concensus about that.
My ukes necks have mini thickness.
Realy.
And fingerdboard thickness is less than 3 mm.
Sometime, I use cedar wood, light and not so dense.
All that not the key for neck stiffness ...
So I put a stringer.
An ultra strong Ø6 mm carbon tube, epoxy glued.
Not only bending come with years on a uke neck.
Twisting is also possible.
Ckeck your few years old ukes...
See on my blog about stringer neck if you have a little time.
I have noticed a little sustain gain with my stringer neck.
Not trussrod - just stringer.