Why do today's musicians find some baroque and classical period music very difficult to play even on modern instruments?

Is this an issue about instruments or about the training of the musicians? I’m not even sure it’s a valid statement. Perhaps, if so, it has something to do with the tempering systems used in modern instruments. Can you cite some reference to support your statement about today’s musicans… difficult to play…?

A lot of this has to do with the specific nature of the instrument. For instance, the brass instruments didn’t have valves, so composers limited themselves to the notes available on the instrument, and allowed time in the music for things like changing crooks to make the trumpet from a B-flat instrument into an A instrument as necessary. Even modern orchestration books will give advice on this.

Modern instruments make more things possible, but they don’t necessarily make things easier.

Baroque-era stringed instruments didn’t play as loud/dynamically, and I’m pretty sure the tension on the strings also differed from current stringed instruments.

As far as woodwind instruments are concerned, the “Boehm Sysytem” of air hole placement, padding and fingering mechanisms was not fully developed until 1847. Virtually all modern instruments use this system. The instruments made before this were much more difficult to play. The “Badinerie” from Bach’s Orchestral Suite No. 2 is probably the most used “tryout piece” by those evaluating orchestral flute players. I don’t think it would be playable on a flute of Bach’s day, but, of course, it was. Mercadante wrote 6 flute concertos before 1847. James Galway, using the best modern equipment, has said publicly that he can’t play parts of them because they are so difficult. Franz Von Suppe and Carl von Weber both wrote extremely difficult Clarinet pieces (e.g. Weber’s “Grand Duo Concertante” for Clarinet and Piano) before 1847. I don’t think a modern concert artist would deny that she probably couldn’t play them – at least as written – on a period instrument. I am less knowledgeable about brass. Playing Bach’s Brandenburg Concerto No. 2 on an ordinary valveless period trumpet would be very difficult. The notes may be written so they “fall easily”, but playing the high notes and sixteenth notes without valves? I think most modern recordings are made playing a trumpet the size of a child’s toy so that the high notes can be reached more easily and the short notes at the proper tempo. Of course, people are playing period strings so my hypothesis doesn’t apply..