The magneto cannot generate a spark when turned slowly
backward, and it's not just because the impulse coupling doesn't work
backwards. The magnet could generate a flow in the primary winding,
but the points are closing rather than opening at the right time and
the arresting of the flow, a requirement for spark, isn't there.
Further, the distributor is geared to send a spark to a cylinder based
on a particular direction, and turning some reversible mags backward
can generate a spark but it happens when the distributor finger is in
the wrong place. Besides, the mag has to be turning at a good clip to
make any spark without the impulse coupling.
Worn-out vacuum pumps could indeed fail if turned
backward, but they don't belong on the airplane anyway. We use the
pumps with the wear inspection ports so that they never get to that
vulnerable stage. A good pump can be turned backward without fear of
breaking anything.
So we teach our students to turn the prop backward, but
that it must be done with great caution as a matter of course. Most of
them don't understand magnetos or engines or anything else and might
someday turn the thing in the wrong direction, and you have to make
such rules so that they learn that props can kill and so they will
keep their friends from fooling with it. We never lose a vacuum pump,
have never had an inadvertent firing. The most dangerous time is right
after the engine is shut down, when hot carbon in the cylinder head
could fire any vapors still present. We also check the mag grounding
at idle just before mixture cutoff.

Dan