From the Zurich Hochschule teacher, Liisa Tamminen:
I was today denied an access to Deutsche Bahn’s bus from Munich to Zürich. I was carrying my viola in the smallest possible shaped case. I offered to put in on my feet, or even take it out from the case, but the driver was only shouting me repeatedly that musical instruments are not aloud in Deutsche Bahn buses, doesn’t matter how small they are. She could not tell me any reason to this, except that that’s a rule. There were many other passengers with bigger and heavier hand luggage.
I booked the ticket from the Deutsche Bahn site, where they offer this as an alternative connection in the same page/time table where you can buy their train tickets.
Luckily the person I spoke with in the Munich main railway center after this incident was very understanding and I could change my ticket to the train, arriving few hours later to Zürich.
I have sent feedback to Deutsche Bahn but haven’t yet heard their answer.
Be careful on DB buses.