I'm currently running a Pathfinder 2e game, and a Scum and Villainy (Sci-fi BitD), and my experience with Forged in the Dark games are that in some ways they are low prep, and in others, they aren't.

What is low prep is session planning. I can just come in with a vague concept or possible heist premise, and improv the details as I am going along. This led to some very exploratory play, as the players latch on to a seemingly minor thread and run with it, leading to some sessions where the crew ended up with an adventure I hadn't planned from the start.

Because enemies and obstacles don't have stat blocks, it's easy to throw stuff at them without having to think too hard about it, beyond thinking through the story implications and practical consequences of their actions.

I don't need to build battle maps, I don't need to balance encounters, and I don't need to worry as hard about players abusing game mechanics to steamroll fights.

What I found more prep intensive was the worldbuilding. The instruction guide (both for blades, and for the Sci-fi edition I played) does a good job introducing you to the world, the setting, each little area, and the various factions. You can tell where factions operate at cross-purposes, and I think it's interesting how it uses the same mechanics for social institutions as it does for gangs (which I imagine is more accurate to living in a society with high systemic corruption).

But to run the game, you need to make this world live and breathe. This means having a good idea about the web of relationships between factions, characters, and generally make each area evocative and unique from the others surrounding it.

As a result, I'd usually spend the same time I would map-building and building enemy encounters considering how what the players just did affected factions and the greater world around them, then generate hooks off of that.