Awesome! Could you share a few screenshots?
This is expected. If spacing along any axis is significantly more than spacing along other axes then essentially your voxels are shaped like sticks, so you cannot represent arbitrary 3D shapes with them. If spacing difference along any of the image axis is more than a few ten percent then I use “Crop volume” module to resample the input volume to have isotropic spacing (and crop it to the region of interest to keep the volume size small).
Tetgen is mainly added for reference, because 5-10 years ago it was a popular mesher and we wanted to make it easy to compare. It has restrictive license (you need to buy a license for commercial applications) and not robust (can crash quite easily for complex meshes that come out of medical image segmentation), so I would not recommend using it.
There are many other meshers, some of the popular ones include netgen, gmsh, TetWild (see this page for lots of more - not very up-to-date but gives an idea how many meshers are out there.). As far as I know, @Sam_Horvath plans to add some more meshers to SegmentMesher.