File system Minifilters can be very simple to write. However, File System Minifilters are particularly difficult to write correctly. This is because many of the interactions that they require with the file systems are entirely undocumented and only learnable through experience. So, not even every Windows kernel mode software engineer has a grasp of the complexity of this field.
Because of the degree of specialization that’s inherent in the Windows File System space, it is a mistake to assume that software packages authored by large, successful, and/or reputable companies are reliable, well-written, or automatically likely to be interoperable. Corporate success or even engineering competence in other specialties is absolutely no indicator of interoperability in this space. There’s a cloud storage solution built by a major vendor that is built upon a framework that is well-known within the Windows File System community to be “less than reliable.” This software doesn’t work with FESF, and unless the underlying framework is dramatically changed (or the software is re-written) it will never work properly with FESF.
When you combine these factors with the fact that there is no “quality gate” that a third party must pass to be allowed to ship their product, the result is a lot of very poorly implemented Minifilters. These Minifilters show their poor engineering by breaking things in the system. Like FESF.