I think that the 2 most flawed missions from Hitman season 1 are Marrakesh: A Guilded Cage and Bangkok: Club 27. The thing is though, I think the big issue A Guilded Cage has is that it suffers when you have to run between two locations on either side of the map, both of which are aren't that interesting compared to the big city space inbetween. So much so that I think A House Built On Sand more or less fixes the issues A Guilded Cage has.
Bangkok is different to me because while I do think it is also a significantly flawed level, unlike Marrakesh I don't think it has one big obvious problem, but rather it has lots of smaller issues. And despite having played Bangkok enough to reach level 20 Mastery - havn't attempted Pro Mastery yet - it is a level I just don't enjoy compared to the rest.
It feels very... blocky, which is a very unspecific and bland complaint, I know, but I find it difficult to really understand why I don't like the level so much. I think it looks gorgeous, and I think Thailand is really an inspired choice, one that you rarely ever see in video games. But at the same time, I just don't like navigating the hotel, be it the upper floors or the basement areas. There is a lot of tight spaces and I just don't get much enjoyment of navigating the place, often more frustration really.
I'm not a fan of the targets either, at least in terms of gameplay, their personalities are fine. I think Jordon Cross especially is a target that isn't that fun to take out, and again, I struggle to really understand why. I like taking out nearly every other target in the game, but Cross feels like a chore to me.
I don't know, perhaps someone who understands more about level design than me might be able to explain why Bangkok could be better, or perhaps say why it is just me who has an issue with the level.
I can think of some small ways to improve the level, like having all 12 standard hotel rooms in usage and each contain something, even if it is just an open window to use as a shortcut, rather than some of the rooms being completely useless. I also think that maybe having the 2 top floors connected by something might of worked too, or at least give us some decent sniper positions.