Yes both acoustic guitars & voice are much more difficult to record than electric ones 
I went on a series of experiences on my acoustic piezzo , I came with very good results after lots of experiments, I use a combination of NAM martin IR's ( free stuff) , the baldringer preamp of the kemper and some UAD post prod.
you can check it there on my video , but yes fresh strings are mandatory !
regarding the voice , it's complicated too and very dependent on your voice type , I use post prod UAD plugins & verbs for great results, while my vocal go thru the kemper , using a voice preamp preset as a voice input, great control.