Thoughts on the most difficult instruments for speakers to reproduce?

String sections, and yes violin higher registers.  It seems almost impossible to find sound reproduction that captures the combination of texture, bite and silkiness of real strings.

And most speakers tend to thin out the body of the sound as you move into the higher frequencies.  Drum cymbals in real life are big and round, never the tiny pin-points of bright sound as through most speakers.

Piano...yeah...always on the list.

I would say piano for all of audio, but violins specifically for digital audio, especially massed strings.

The piano has the widest range off all instruments !
https://spinditty.com/instruments-gear/Why-the-Piano-is-the-King-of-Instruments

1) Without doubt a realistic live drum set. Percussion has such massive dynamics most speakers badly compress at realistic levels.

2)Grand piano is tough too.

3) Pipe Organ - especially the low notes.