Cowboy job !

I wouldn't have thought it safe ..surely there should be some brackets ?

Why on earth can't you demand they do it better? It's not installed properly or professionally and may fall!

see, I didn't knew about the brackets, I looked closely and it seems to have them on 3 sides, only the front doesn't have anything

Is that a cast iron sink or stainless?

stainless, made by Hahn.

this is overkill but it's the way the pros do it for heavy cast iron sinks. It can be mounted in place of what you have. https://sinkmount.com/product/petes-sink-mount/ good luck

That sink's a leaker, I promise. Since the thread's a year old, I predict there is brown gunk at the joint between the flange and the bottom of the stone already.