The big advantage of that method is that the sandwich skins are more
stable, and less prone to mirroring the spar after aging. One of the
disadvantages is that the spar is less stiff than it would be if it
went from outer skin to outer skin, and didn't have its depth reduced
by the sandwich foam.

I don't think this is a disadvantage, but is instead an advantage,
because the wing isn't so stiff, and the pilot enjoys a smoother ride
in turbulence. The disadvantage is it likely takes more material, so
the wing is a bit heavier and more expensive.
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I don't know Eric, that flexibility and smoother ride might be your
upper spar cap lifting off of the shear web. There is no law which
says the wing has to actually snap. It could be more benign; one wing
a little more flexible than the other or air brakes popping open
during high speed runs in turbulence. Each 4g pop might add another
..010" to the delamination between those two air bubbles in the bonding
paste.

The stiffness, or lack thereof, can still be designed into the wing by
varying the amount of carbon fibre rovings in the spar cap, or
Graphlite rods in the case of the Genesis 2 and LAK 17. I believe the
LAK 17 wing has the externally made spar bonded to the outer skin of
the wing because the designers wanted an extremely thin profile. The
spar is only 3 1/2 inches deep at the wing root of the LAK 17; bonding
it to the outer skin of the wing allows the wing to be extremely thin.
Unfortunately, some of the earlier 17's had that mirroring and the
factory paid to have the wings re-profiled. I believe that happened
to some of the earlier ASW 28 gliders, too. Still, the use of
Graphlite rods instead of carbon fibre rovings, and externally built
spars which are then wrapped in fibreglass cloth and vacuum-bagged is
a method which produces stronger, more durable and longer lasting
wings. As the current fleet of German gliders gets older I'll wager
there will be a lot of speed limiting directives because the "margin
of error" isn't what they thought it would be.
What kind of spar construction methods do the SparrowHawk, Apis and
Russia gliders use?