You can do two things to workaround the issue that comes up using fully threaded screws (that it is difficult to fully clamp the pieces together and they can potentially be pushed apart):
Clamp the pieces before screwing - this will ensure they are held tightly together.
Drill a larger pilot hole in the top piece. This prevents the screw from threading in the top piece and ends up working the same way that screws with a plain shank near the top work.