Articuno is a pretty defensive Pokemon, so for a defensive Pokemon, losing half your health upon switching in is a pretty big deal. Also, its typing isn't that good defensively. Weaknesses to Fire, Rock, and Electric, which are fairly common, really hurt its surviviability. Also, Articuno only resists Bug and Grass, and is immune to Ground, but that's it. So it's a lot easier to kill Articuno than those defenses and HP might make it seem. That's really the main problem. Offensively, 95 Special Attack and 85 Speed aren't horrible, but they aren't outstanding either.