I love the viola, but it just isn't a great instrument for virtuosic repertoire
Every viola concerto I have listened to, when it comes to the virtuosic passages like fast double stops, spiccato, etc. the viola's register and warmer timbre just struggles to project over an orchestra. Even with great professional soloists playing, you just end up hearing the bow scratching against the strings since the timbre of the instrument blends too much with the orchestras harmonies. Don't get me wrong I love the viola, especially when it comes to lyrical melodies. My favorite types of pieces in the viola repertoire are more on sonatas than concertos for this reason since there is more emphasis on melody and developments than showy virtuosic fireworks like concertos usually are.
Feel free to agree or disagree in the comments, I would love to hear your insights.