I don't have much experience with Dwarven Forge, but I know the writing on a mat to draw a map is the least time intensive part of making a scenario. Deciding on story, description of areas, character names personalities, terrain and obstacles, enemies and combat setups, and all of the other stuff is massively more work than just drawing some lines to show room sizes. Even deciding what an area looks like and how it's connected is much more time than the physical assembly.
All of that is done before play starts. During play is what I'm referring to, when the party goes off the map or out of the pre-built set-up.
The same thing happens in our online play: we-as-players know the DM might have spent hours setting up the map, which puts meta-pressure against our going elsewhere even if it's what the characters would otherwise do.
I wouldn't expect someone to build the not-the-map part of a scenario on the fly at a convention game in a narrow timeframe with players they haven't met before even if they had helpful elves who could instantly reconfigure any terrain in seconds, that's not the difficult piece.
I'm talking about home games here. Convention games are a different animal, where by sheer necessity things have to be limited due to the tight timeframe. For those, doing up a fancy diorama of the complex where the whole game takes place could make loads of sense, and add to everyone's fun.
But for home games, a DM isn't going to want to spend hours building an elaborate set-up of a castle only to have the players decide to explore the outbuildings or surrounding woods or nearby caves instead.
If I'm running a game with open sign ups, there's a hard requirement to stay within what I've prepped and advertised that I'm running. I wouldn't call it a 'certain amount of out-of-game pressure', that understates that if you don't want to play the game you signed up for, I'll tell you to leave the table. As would every other DM I know, and I know a lot of DMs who run at least some games in a 'signup' format.
Again this sounds like convention games. Home games are (I hope) considerably more malleable over time and might well jump all over the place as the campaign goes on. Sure you say you're starting with Keep on the Borderlands but that doesn't (I hope!) mean the party are committed to staying there should things in the game suggest otherwise. Example: the party get to the Caves of Chaos and get thrashed on arrival, the survivors go back to the Keep then decide "Screw this, we're going to seek adventure in the lowlands as this is just too bloody dangerous!"
Player agency being exercised, that is.
Do you really sign up for a game that's advertised as running "The Sunless Citadel" and then insist on your characters spending the session shopping in Waterdeep or sailing out on the Moonsea as pirates or wandering off into the woods looking for fey instead of going into the sunken keep? I've literally never encountered that behavior, and I'd probably think someone doing that was either mentally unwell or trolling, it's like going to a book club and insisting that people play baseball instead.
If I join a game and the DM says "I'm running The Sunless Citadel", I would assume that to merely be the starting point for a much bigger campaign, meaning after Sunless (or even during, if things go badly) we-as-characters can go wherever in the setting and find other adventures. If that assumption is wrong and Sunless is in fact intended to be the entire campaign then I wouldn't even bother joining.
In fairness, I'd also warn the DM that I'm familiar with Sunless Citadel in case she wants to modify it or choose another.
