Disadvantages over Traditional XP in Lineage2
There are three big risks to AoE parties: Mobs, Bufs, and Players (Lineage 2 gold). The Mobs part is fairly obvious: if your tank does not take enough 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. Loosing bufs can be one of the worst things that happen to an AoE party.
This risk is lessened in C4, due to buf timers so you know when you need to rebuf, but it is still very important. A cancel at a bad time or fallen bufs will more often than not lead to the death of one or more party members. Lineage 2 is a PVP game (Lineage 2 money), 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.
AoE parties often involve the use of AoE skills, and hostile players can use that to their advantage. A flagged hostile player can move into the kill zone of the enemy. This causes the AoE party to flag when they use their AoE skills, leaving the party vulnerable to PVP. Often, players will only kill one or two key members of the party, leaving the rest of the party to die to mobs in hopes that gear drops. Most AoE parties consider tactics like this most despicable, but the enemies of AoE parties can use this to great advantage.