Regarding the failed parts, from the rust it looks like 4140 or another steel with a carbon content allowing it to be hardened was used, but the part may not have been hardened. (Low-carbon steel tends to have more pitting, hardenable steel tends to have less pitting, so to me the rust suggests it's "probably" "okay" steel.)
The design seems fine for the task as long as the coupler is not being used as a U-joint and is greased. Regarding "slamming," remember the clutch should be engaged slowly at low RPM, so the coupler should absorb the minimal shock load just fine except in -35°F = -37°C or colder temperatures. (I'm a Design Engineer in the steel industry, though not for snowblowers, toasters, or your failed coffee pot.)
If the alignment stud was drilled off-center as the pictures might suggest (the pictures could have been taken at an angle and doing so could result in a similar looking image) the off-center alignment would cause significant wear and rapid failure.
I'd first investigate the alignment, then the steel hardness. Even without grease, off the top of my head I would guesstimate a properly manufactured coupler should not have failed in under 300 hours of use.