Shuckle's claim to fame is having the highest defense stat in the game by a fairly good margin. That said, even at level 40, it ends up at 79 HP and it's attack stat is pretty abysmal. It's sky-high defense means that incoming attacks do significantly less damage, but when you combine that with it's low HP, it's overall bulk ends up being nothing too special. And with it's low attack stat, it will hit back with the strength of a wet noodle.
It makes for a cute novelty defender with a very low motivation decay rate, but that's about it. Unless you really love Shuckle, I wouldn't recommend spending dust on one as it also won't be a viable option as a gym/raid attacker or in PvP.
