The main disadvantage of the Duron is that it has a much smaller L2 cache than the P3 or P4 - 64K compared to 256K or 512K. On the other hand, it has a much bigger L1 cache - 64K each for code and data. The P3 has only 16K each; the P4 has just 8K for data and a 12k-op "trace cache" for code.
The smaller L2 cache makes it less effective as a server chip, but it's still pretty good. It would make a decent web server; I'm not sure about game servers.
The Athlon, Duron, P3 and P4 are all solid general-purpose chips. The only current x86 chips to really avoid for server use are the Via C3 and the Transmeta Crusoe, which are considerably slower.