what seems to work: draw a spline to follow; draw a closed profile; position the profile near one end of the spline; rotate the profile to be approximately face-normal to the spline i.e. face normal aligned with tangent at near end; sweep. .
if the profile is not near the spline it seems to swerve oddly and the resulting tube isn’t shaped like the spline, especially at the beginning. if the profile isn’t close to face normal, i often get a message about intersecting geometry.
is this expected? what are the rules?