For me, the hardest out of all the ones I’ve learned was the guitar. I think it was because of the lack of keys. I also know how to play the clarinet, flute, piano, trumpet, saxophone, and mellophone. All of those have keys which I think makes it a bit easier. With the guitar, you have to get a feel for the frets and be able to tell which fret you are at. Yes it can be done, but for me personally, I think it was the most difficult.
I’d guess that trombone would be a bit more difficult for those same reasons, since it goes by slide position instead of keys and if you are off on your slide position, it affects your tone.