Having worked on that code (in Packer) - the WinRM protocol is completely unsuitable for use as an SCP replacement. It's amazing it works at all even for small files! The actual "winrmcp" implementation is here https://github.com/packer-community/winrmcp if you're interested in the inner workings.

Yep definitely the worst thing ever. Not just that though; everything. I've lost so much hair in the last few months over trying to get a CD environment up on windows.