Drapion can be really good at sweeping, but seen feom my experience, he does have some points to concern that cannot be left unattended, however, it's better to address the pros first in case you got scared away by the cons:
1. Drapion has a "Golden Balance" or so my own creation. Drapion does have high defence, but at the same time is fast and hits hard enough. For a sweeper, I'd say is perfect, as most sweepers are REAL frail. Where most of your sweeper brethen fails, Drapion might have a chance to survive and succeed.
2.Drapion has a terrifying ability, Sniper. It increases the damage dealt of critical hits by 1.5. If that's not scary enough, it can learn High Crit Ratio moves. Plus give it a Razor Claw or Scope lens. If possible, it's Damage Output is skyhigh and sometimes OHKO unexpected OUs. And oh... Put in a sword dance if you feel like it, just to make it worse.
3. Versatility. It is known that Drapion excels in being a wall, but put in a few offensive moves really make it shine.
But, the cons:
1.Weakness to Ground and Steel Types. It should be noted that most teams have at least one Pokemon thats a steel type or have Earthquake. As much as Drapion's offensive capabilities, steel types are immune to poison type attacks. Luckily, Drapion have access to Fire Fang and Aqua Tail, but most cases it is a 2HKO or more.
2.All about luck. Although you have really high crit ratio, it doesn't mean every hit is one crit. All depends on luck... But if you're lucky then you might just take out at most 4 foes (personal experience in OU).
3.Weak in SpD. Drapion does have great Defence Stats, but really lacks in SpD. Stats. One Bulldoze could take it out... But most just go for Earthquake, although it is something to be noted of.
Overall, Drapion is one of is kind when it comes to being a sweeper, where others fail, it succeed. (At times not being OHKOed), but also have some deadly flaws... Use at your own risk, but it's rewarding when you succeed.