Well, that's a funny thing. The "well engineered" code can actually be harder to work on. And maintainability is one aspect of "well engineered".

What I've seen (at least at one place I worked) is that the code may be very well engineered for further modification - but only by people who have worked quite a bit on that code. For someone coming from outside, it's almost impossible. It takes forever to find your way around, and figure out where the magic place is that you need to modify in order to work within the existing design.

Well, you say, that's what the design documents are for! But they didn't have enough detail. They described the major objects, but not all the ancillary helper objects. They didn't describe the flow of data through the system in enough detail to know what to modify. The knowledge was there, in peoples' heads, but it was hard to get those peoples' time.

So, yeah. "Well engineered", but not engineered for maintenance by people who are not those already in the group.

You're describing something not fit for purpose. It's not well engineered.

That's like saying "that's the irony of it, a well engineered bridge sometimes drops chunks of concrete onto cars that pass underneath".