The biggest drawback I focus on the cost side, and then in practicality.
I mean that often similar equipment that are in use and have actually completed their life cycle and implementation of accessories analog-digital conversion (which are available) have costs that do not merit the implemntacion on those computers already several years of use.
It happens that on many occasions it is more convenient to total depreciation of the analogue system once and for all and make an implementation and based on new technologies and cloths not apply warm water to more "problem".
An interesting case of migration may be the Medellin Metro, where last year he migrated all analog technology in audio system that had implemented 15 years ago to a new one based on digital transmission. for this basically they replaced all the electronics for amplifying equipment and new routing, but retained the original speakers.