It's a lot better than people think fusion rockets would be in real life. But the writers actually acknowledged that, in the episode that goes into the drive's history. They had fusion rockets before Epstein, they just weren't near as good before the breakthrough.

And most of the story wouldn't even change if it took longer to go places. The only difference would be that they couldn't show normal gravity inside a spaceship and attribute it to acceleration. It's a slight distortion that saves them money on special effects.

They especially don't show long travel times, but in the books travel is usually counted in weeks or months. Also, in the books they usually travel at ~0.3g for the sake of Martians and the Belters, who are mostly not comfortable with Earth's 1G.

(This is not merely a detail, it's one of the core plot points of The Expanse, as tolerance to gravity is what makes Martians and Belters almost a different species from Earthers; the former can not live on Earth, and so they don't care much about it.)

Any advanced technology... :)

I don't actually recall what the in universe explanation of it was outside of it running off water as its fuel. Either way its specific impulse is crazy.

The authors intentionally made it the one thing they weren't going to try to explain using known physics. It's reactionless and fusion powered.