Heya, thanks for the feedback. Just to be clear, the add-on doesn't do anything regarding symmetry when making weight paint strokes, but yes, you should be careful with symmetry options in Blender because they can be wonky sometimes. There are however operators to symmetrize weights, and those work to an extent even on not perfectly symmetrical meshes.
Great addon, but make sure your mesh is mirrored to use this tool!! I had issues where the weight painting wasn't properly mirroring, especially for clothing. I also wish the "Auto Cleaning" feature was off by default.