It's true, the Enterprise-D is unusually fragile in the warp drive, everything from warp engines to warp core and between. I write, unusually, because no other ship shows that much sensitivity. Even the Odyssey in DS9 takes a shot to the nacelle the very first moment it gets into battle, in "The Jem'Hadar" yet does not explode, she only has a plasma leak, and continues fighting, shields down for something like 10 minutes. Her performance is vastly superior to that of the Enterprise-D in
Generations.
Then there was the warp core reaction chamber flaw which in "The Drumhead" was used as evidence for some sort of sabotage, even though it was just manufacturing or design error. But, being a manufacturing or design error is extremely serious in its own right.
In "Cause and Effect" the Bozeman
hits the Enterprise-D nacelle to nacelle. Yet it is the Enterprise-D's nacelle which explodes, and causes a warp core breach, while the Bozeman's nacelle is fine.
In "Timeless" Voyager
crashes belly first, hard, into ice in such a way one of the nacelles must have hit too. Then the port nacelle hits an outcropping is torn up. Yet, Voyager does not have a warp core breach, and despite the nacelle glowing before the damage it did not violently explode on impact at either point in the crash.
On the good side of things, the writers or producers seemed to be very aware of what they were doing in making the Enterprise-D fragile. I won't speculate as to why, but the issue crops up in ways which cannot be accidental and which tell a story in their own right. It's kind of the same thing with how Picard and Riker disdain the idea of combat as uncivilized and perhaps obsolete, early in the series, and entertain the idea only for the mental exercise of it. But by the end of the series they have fought so many times it is hard to think of them holding exactly the same beliefs. In a way it's an metaphor for the Federation as a whole.
