Traditionally you have error correction issues which require frequent and specific manual tuning on a case by case basis. This was the age of the analog machine

Traditionally you have error correction issues which require frequent and specific manual tuning on a case by case basis. This was the age of the analog machine. However there are some recent works which have introduced compromises to continue to do just that: https://www.nature.com/articles/s41928-018-0100-6 https://www.nature.com/articles/s41928-018-0100-6