I've taken trains in Italy, and I've taken FlixBus from Slovenia to Trieste. I don't really like buses and prefer trains when they are practical, but otherwise, my FlixBus ride was fine - a little late (nice to have the FlixBus app on your phone - you get notices about updated arrival times, etc.) but comfortable enough, for a bus. If there had been a train running at that time, I would have taken it instead, but at the time, there wasn't. If train vs. bus in general doesn't matter to you, the bus would probably be fine.
One benefit of taking a bus to Siena instead of a train is that the bus should drop you right at the edge of the old town whereas the train station is a little further from the center as I recall.
You can buy FlixBus tickets directly from the FlixBus website or their app, FYI.