How realistic is atc-sim.com as an ATC simulator?
Not very realistic.

it's text-based; no readbacks.
there are no approach clearances. You can assign speed, altitudes, and headings, but there is no way to instruct an aircraft to execute a specific approach.
there aren't, really, any missed approaches or go arounds
no emergencies

Basically, it' just a game of &quot;keep the aircraft separated&quot;, but it doesn't properly simulate how ATC works.