I say all this as someone who enjoys NMS, has about 80 hours in it, and there is absolutely fun stuff to do, and is definitely worth playing. Having said that, it is very shallow for the following reasons:

NMS has very little story. I've played through all the missions, and there is very little dialogue, very few characters, virtually no choices. So overall the story is very shallow, but it's fine.

The random NPCs say very little, and do almost nothing. So quite shallow.

The space stations have very little variety, and very little life to them. Same with settlements and trade centers on different planets.

The randomly generated structures take on maybe a dozen varieties, so they get extremely repetitive after the first few hours.

Combat is very limited and repetitive. There's only a few enemy types, and pretty limited weapon variety. Same with space combat, which pretty much always plays out the same due to limited weapon variety and limited ship controls.

But again, I like NMS, and do recommend it. It's just not a deep experience.