Most problems with the quality and consistency of buttonholes can be associated with the the feed of the fabric because the long buttonhole foot is not completely level at all times. Even a slight bump from stitching or a piece of seam allowance can tilt the foot and reduce the traction of the feed dogs.
That's why the vintage buttonholers work well. Lower the foot lever with a snap and the whole of the fabric is held securely and moves with the attachment rather than the foot moving over the fabric.