The problem with these dragons is the unclassic spawn time they have.
They should spawn every 7 days on the dot. Here they spawn every 7 days +/- 8 hours.
Using Nagafen as an example. He will spawn some time between 4PM Sunday - 8AM Monday. So there is a very high likelihood he will spawn while most people are in bed before their Monday work week begins. Making scheduling a raid impossible.
More than the top 2 guilds get these mobs. But it's only guilds who actively track or quickly show up on a server reset who get them. They die quickly after spawning, never just sitting up.
http://www.project1999.com/raid.php
That page explains the classes.
The 52 rule does apply here, they will banish players over 52.