WOW PowerLeveling: There are three big risks to AoE parties: Mobs, Buffs, and Players
Disadvantages over Traditional Experience: There are three big risks to AoE parties: Mobs, Buffs, and Players.
The Mobs part is fairly obvious: if your tank doesn't take enough WOW PowerLeveling hate, or your healers start healing too early, or one nuker over-nukes another nuker, a key member of the party will die, and the entire party will die.
The best way to counter this problem is to make sure everyone knows their place in the party.
Losing buffs can be one of the worst things that happen to an AoE party. This risk is lessened in Chronicle 4, due to buff timers so you know when you need to rebuff, but it's still very important.
A cancel at a bad time or fallen buffs will more often than not lead to the death of one or more party members.
Players - Lineage II is a PvP game, and that means all the time. Players can ruin an AoE party faster than anything else. A silence or cancel to the healer in a clan war, a PKed tank, or the loss of a damage dealer can all spell doom for the party.
Most AoE parties consider tactics like this most despicable, but the enemies of AoE parties can use this to great advantage.