1. A lot of seeders doesn't mean good download speeds. Some are
behind firewalls, especially both hardware and software firewalls,
which can greatly reduce the speed.
2. Most ISPs limit upload speed to prevent abuses, in this case
P2P.
3. Seeders may not be active.
4. Your download speeds is very much affected by external factors,
including the things you do. If you are downloading stuffs and is
using a download accelerator, don't expect your download speeds to
be anywhere near great. You only have this amount of
bandwidth.
5. By using P2P, you are already at risk at being hacked. You are
connected to lots of unknown people's computers and anyone can do
anything to it.