This is a weak metaphor, but a digital monitor can reproduce a fine work of art, but synthesizing (or modelling) the look and behavior of real oil paints digitally is a challenge.
Likewise, getting the algorithms just right and in the right orders and quantities to replicate everything analog modular synthesizers can do is very tricky, though theoretically possible.
Likewise, getting the algorithms just right and in the right orders and quantities to replicate everything analog modular synthesizers can do is very tricky, though theoretically possible.