Has anyone ever taken a Fix bus in Italy? Are they safe and reliable?
The company is known as safe and reliable, but I would only consider buses for the areas where trains don't provide much direct service. That may be the same criteria where Flixbus has chosen to operate in Italy - around Calabria, for instance.
Be sure to compare travel times between bus and train. The trains will usually save significant time.
Flix bus is fine
The more popular routes Rome to Siena and Florence to Siena are both direct faster than the train.