What Is the Cause for Poor Results with Adaptive Thresholding?
Some guidelines for using Thresholding:


Stretch the image to use the whole Dynamic Range (DR).
Apply some Denoising (Very very gentle).
Median with small radius would be a good idea.
Unless you hand tweak the Threshold, Otsu's Method generally yields good results for this kind of tasks (Text on background).
If one use Adaptive Local Methods (Mean / Gaussian based) the Radius is another hyper parameter to tweak.