The Shadowed Obelisk breaks one of the basic tenants of a West Marches campaign: At the end of each session, the PCs must be back in town.
The most obvious reason for this is that it keeps PCs from being "locked" in an adventure and unable to be played if last week the assembled players didn't finish the adventure. As this week is likely to have completely different players or one (or more of) the players available last week is not available this week (or for longer), PCs out in a dungeon are in limbo that is fairly disrupting. Not only can those players not play those characters, but other characters wanting to explore that location are either kept from doing so or strange continuity issues can crop up when the first group finally resumes the adventure.
There is also a more subtle reason for this tenant: players might be in a different mind-set later and want to do a different style of adventure session. By having each session end back in town (or an accepted waystation), the players can choose adventure locales that match what they want to do that night. Rough week at work and want to blow off some steam smacking down monsters? Let's go to the Terrace of Fallen Horses and beat down some undead. More interested in exploring? The Underfortress still has plenty of space still unexplored and you can go as deep as you want and then leave.
The Shadowed Obelisk locks the PCs into the location until they find the way out. At the current rate, this will take another session or two, depending upon how unlucky the players are. One of the players available in Part 1 was not available for Part 2 (due to his wife's birthday) and another vice verso (he runs a Shadowrun game every other week at the local college gaming group meeting). Due to the nature of the twisted time and space of the Shadowed Obelisk, I was able to hand wave the character switch and still keep it internally logical, but in, say, the Underfortress, where normal fantasy reality applies, it wouldn't work without a major drop in suspension of disbelief.
It is also depriving the players of choice in what they want to do for the session, something vital to any campaign, especially to a West Marches style campaign. My players have been significantly more proactive in this campaign as opposed to my Naze Valley Rangers campaign, where I tend to provide them with adventure hooks and see what they do with them. I like the sandbox proactiveness and want it in more campaigns. Not allowing multiple sessions in a single location erodes that and I don't like it.