Is there anything especially hard about decompiling (to) Java?

.NET/C# decompilers are widespread and generally work well (there is one built into Visual Studio nowdays, JetBrains have their own, there were a bunch of stand-alone tools too back in the the day).

< disclaimer - I wrote CFR, which is one of the original set of 'modern' java decompilers >

Generic erasure is a giant pain in the rear. C# doesn't do this. You don't actually keep any information about generics in the bytecode, however some of the metadata is present. BUT IT COULD BE FULL OF LIES.

There's also a huge amount of syntactic sugar in later java versions - take for example switch expressions.

https://www.benf.org/other/cfr/switch_expressions.html

and OH MY GOD FINALLY

https://www.benf.org/other/cfr/finally.html