Lemmy isn't a perfect solution. The problem with Fediverse technologies is that they're not actually decentralized, they're polycentric. As you pointed out, an instance admin can make a lot of trouble for the users of their instance.
However, it is a step in the right direction. The cost of switching from a particular instance to another is fairly low. Your point about reposting supports this argument. If other communities offer the same content, there's no need to tolerate bad administration.
I don't really see reposting as a serious problem. It's a minor annoyance.
Privacy, I don't think the privacy concerns are more serious with lemmy than with something like reddit. Just don't any personal information to your account if you can avoid it.
Server issues, I don't know what the solution is to that.
I don't think dry air makes you sweat less. It just makes the sweat evaporate faster, which also makes it more effective at keeping you cool.