90% at least, most of them are admins screaming for help instead of searching for solutions :)
Both were stolen at different times, from parts of the game development team from inside NCSOFT, these files are known as "leaked official files" (although you will not find the exact copy of them, many have modded them and only modded files are out there, and many of them are backdoored)
Big, Epic, -WRONG-. L2J Servers are only unstable because of the loads of crap all the server admins put on it. If you know how to code in Java you can sit down and optimize the code in order to use less system resources, and advanced functions for handling. A developer who has done that is/was one of the L2jFree team, NB4L1. Some time ago he said he reduced the startup load memory to 256mb (When now you most newbs server admins give 1024 - 2048 memory to their servers ON STARTUP not on high server load)
I've seen L2J servers last for 3-4 weeks without rr, and even NCSOFT has maintenance downtimes every week.
Many tried, many got bored and went to Java, although I knew of a project that went quite well. (I'd like to see a C++ emulator for that)
L2Retail and L2Off = Lineage2.com
L2Off -BASED- (Key word) servers = Servers based on the official leaked/modded files
L2J = What he said, except the lag part. Again, servers with PROPER Java devs have 2-3k server with minimum lag.
-My Addition-
L2J : Uses MySQL, runs on both Windows and *NIX machines, is split into LoginServer and GameServer.
L2Off Based: Uses MSSQL(Microsoft SQL), runs ONLY On Windows, is split into LoginServer, GameServer, WorldServer, NPCServer. (Which is the main advantage, since on high server loads you split those in 4 different hardcore machines :) )
-/My Addition-
Well, mostly, but is ok :)
Best Regards.