Earlier quoted context omitted.
For a lot of filesystems, growing is indeed trivial and even shrinking is explicitly supported. Why would this be less tested than any other feature?
Because users put their systems under various I/O workloads every day, but very few people resize their file systems and they do it very rarely.
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.