> As practice gradually moved away from this insular model, and standing, virtuoso quartets replaced ad hoc groups as the chief vehicles of chamber music performance, Beethoven’s compositions became increasingly experimental (and daunting). Legend has it that when Ignaz Schuppanzigh—a well-known Viennese violinist and Beethoven supporter—complained about a particularly difficult passage in one of the Op. 59 quartets, the master retorted, “Do you suppose I am thinking about your wretched fiddle when the spirit moves me?”
If you write a piece that a player struggles to play, that will sometimes come through via a muddled and strained performance. There are famous exceptions that prove the rule on this, such as the extremely high register opening bassoon passage from Rite of Spring through which Stravinsky intended to convey strain, or the trumpet squeals of Ray Nance at the end of many Ellington arrangements.
Generally, though, you want the players to be able to play your piece well. And "err on the easy side" is an effective way to do that, especially when writing for instruments you aren't personally an expert with.