I think vocals are by default the most personal and sensitive instrument to most people, both musicians and listeners, simply because we’re all so trained on recognizing different voices, interpreting speech, detecting emotions etc - it’s all considered basic social skills.
And of course, this weird phenomenon on hearing ourselves on recordings…
That said, once you become deeply familiar with any instrument (or Morse telegraphy, BTW), you start to perceive it more like human speech, in that you pick up on subtle details that have little impact on the untrained ear. I think it’s the same phenomenon as speech/vocals. It just seems different, as most of us can’t even remember a time before we could understand words, or recognize people’s voices.
And, either way, yeah, expression is what it’s all about… If you don’t have a clear vision of what you want to express, sampled instruments might save your *ss to some extent, as they tend to have a fair bit of “canned expression,” but with a real (or modeled) instrument, you have to rely entirely on your own expression.
On the upside, if you have a feel for expressive playing/singing/whatever, I think you’re halfway there, regardless of which instrument you pick up. Sure, you still have to practice the mechanics of getting the instrument to do what you want, but knowing exactly what you want to achieve gives you a very short feedback loop to guide you towards the technique you need.