Well it's about the same amount of open as far as licensing is concerned, but it's created by microsoft rather than oracle, so you're less likely to be chased into ridiculous lawsuits.
Downside is: the library ecosystem is less open. Some libraries are proprietary, which is not something I've encountered in other ecosystems. This seems to be changing though, now that C# itself is open source.
Patent extortion works by threatening the most applicable patents you have, then licensing your whole portfolio. If they didn't have fat32, do you really think they wouldn't have just used another patent to try and extract rents?
Downside is: the library ecosystem is less open. Some libraries are proprietary, which is not something I've encountered in other ecosystems. This seems to be changing though, now that C# itself is open source.