One disadvantage of the Windows system is, as far as I understand it, that, once relocation happens the libraries can't be shared in memory anymore. This defeats the purpose of a shared object somewhat.

After the loader patched in the new addresses the pages making up the library contain different data for multiple copies of your library, and differ from the data in the PE file. So the PE file can't simply be mmaped and the same pages can't be used by different processes.

I'm not saying this is a big issue, but I think it's a difference worth mentioning.