Case in point: I tried to shrink an NTFS filesystem offline with ntfsresize (offline!) and this happened:

https://marcan.st/2015/10/rescuing-a-broken-ntfs-filesystem/

This is a tool that is part of most Linux installers and tested by huge numbers of people, and yet things still went wrong. Shrinking filesystems is hard, and this was offline. Shrinking filesystems online is much harder.