It is difficult for me to say how good they really were in Windows, since it didn't matter. We needed them in Linux and we only did tests in Windows to make sure a specific unit wasn't defective. They definitely did behave measurably better in Windows - after comparable amount of testing there weren't any dropped bytes.
There is one additional piece of information however, which can explain at least part of what I saw:
Most of the multiport USB-to-serial adapters are in fact a USB hub with ordinary single-port adapters hanging from it. The problem is that the hub is USB 2.0 while the serial adapters themselves are USB 1.0. Linux kernels at least up to 2.6.26 had a bug where USB 1.0 devices did not work reliably behind a USB 2.0 bridge.
I have complained about this problem before on LWN and I got advice to try other config settings, which I did, but saw no improvement.
I am not 100% sure the problems were caused by this and it is also possible that it has been fixed in later kernel versions.