Boeing and Airbus doesn't have an HSI mode which supports FMS navigation
It depends i guess, on the 737 Classic we had an HSI mode that supported FMS navigation, and even on the 737NG with performance navigation scales it is absolutely no problem to fly RNAV without the use of a flight director. On the bus it is not that easy, but the A320 avionics are a good generation behind the current NG ones.