The first generation arduinos did not have flow control to built-in serial converter, true. But it does not mean "Arduino does not support FC" - if you connect a your own serial port to a "pro" series device, you can implement flow control without any problems.

And all the next generation arduino clones with built-in USB stack are just wonderful for serial data! I went over 3Mpbs on atmega32u4 device, and it came with USB flow control, so speed goes up as USB bandwidth is freed up, and connection pauses if data is not consumed, fully automatically.