I did not know that instrumentation recognition was an unsolved problem. I wasn't the best at recognizing instruments when it came up in AP Music Theory tests but at least I have something in my skills toolbox I can point out as being still superior to that of the machines.
Instrument recognition is mostly solved in the isolated notes or single instrument/monophonic scenarios. The problem that remains is instrument identification in polyphonic songs where multiple instruments occur, and even overlap. Most current research focuses on solving this task.