I was following a CGMA course and the instructor asked to sculpt a hero rock, but since megascans offers huge variety of these assets, i wonder if i needed to sculpt the rock anyways to get a job?
If you’ve been asked to do that, then you probably should 
You wont get any points for introducing a megascans rock. Also, if you are looking for a job, knowing how to make your own assets is more valuble than knowing how to import one.
In addition: Those megascans assets are very ‘no compromise’ in every sense, including resources. They are good, of course, don’t get me wrong, but why is it some of the rocks from the Kite demo are still so incredibly useable, even after all this time. You can blow them up to 5x and they still look good. Very little resources used.