I don't see it as necessarily being poor design, though.
When I design a dungeon-type adventure, unless I'm intentionally designing something more gonzo or "funhouse", I try to put myself in the shoes of whoever first built the place and think "What makes (or made, at the time) the most sense design-wise for the intended purpose of this complex?". And if, while standing in those shoes, I want there to be a hidden area within what I'm building then I'm going to make sure there's only one way in to that area and also make sure that said way in is as hard to find as I can manage, for someone who doesn't already know where it is.
From there, I think about what subsequent occupants (if any) there might have been and what they might have done to the place in terms of renovations, additions, destruction, and so forth; eventually leading up to the complex the PCs actually find and explore.
And so yes, in this design there's a chokepoint (a well-hidden and hard-to-open single entrance to a whole new area) and the very real risk of things being missed. Poor GAME design? Maybe, but I don't care much about that; I'm far more interested in designing something that makes in-fiction sense (i.e. what to me is good design), and it's on the explorers to make sure they take the time to gather information, explore everything, and still be prepared to miss bits of it.