The shear pin would fail at idle. The propeller on an airplane
is the engine's flywheel, and its inertia carries the crankshaft
between power strokes and smoothes its rotation. There's a
reciprocating force between the prop and crank flange, therefore, and
ANY looseness between the two will result in forces that will fail
attaching hardware. There have been cases of six 7/16" prop bolts
breaking when they weren't tight enough. There have been cases of
charring of wooden props when they weren't tight enough; the friction
created by small amounts of movement between the prop and flange heats
the wood. Props use six or eight substantial bolts for a reason: to
keep the thing on there tight. A shear pin in the system requires
looseness in the drive, and it just won't work. This isn't a boat ot
tractor. Imagine the damage when the flywheel of an outboard motor,
auto or tractor engine is a little loose.
Further, a sudden stop of the prop also pulls the crank radially
as the weight of the prop at its free end tries to keep moving. The
crank gets bent anyway, or the case fails, or engine mounts are
damaged. Any forward movement bends the prop backwards, too.

Dan