Honestly I never viewed Excadrill as "paper thin" but I think the fact that he never invests in his defenses and he always has a Jolly or Adament nature that there is an obvious difference between the two pokemon when comparing their defenses. Also, Excadrill has life orb recoil and is weak against 3 priority attack moves (2 common fighting, 1 water).
Also, Reuniclus has Calm Mind and Recover, 2 things which help his survivability substantially. And lets not forget magic guard which takes away chip damage of sand storm or toxic or burn... His Physical Defense is negligible though, but with a bold nature with 252 HP/252 Def, it puts him at 273 defense... about comparable to a neutral 0/0 105 base defense.
Also, Reuniclus has Calm Mind and Recover, 2 things which help his survivability substantially. And lets not forget magic guard which takes away chip damage of sand storm or toxic or burn... His Physical Defense is negligible though, but with a bold nature with 252 HP/252 Def, it puts him at 273 defense... about comparable to a neutral 0/0 105 base defense.