Oh I can explain that one too (glances at technical manual hidden under the desk).
Nerd Alert!
The warp field does cause gravitational stresses on the ship through the process of compressing and expanding space, see diagram 1 in my earlier post. It is basically creating a small gravity well in front of the ship to "fall" into continuously with the opposite at the rear of the ship to simultaneously "push". The ships structural integrity field is supposed to counteract these stresses in flight, but it does have a limit.
I'm sorry I ignored that in my earlier response which was focused on why the nacelles don't produce thrust. They can be torn off by having too strong a warp field, which would create an extreme difference in the gravitational pull on different parts of the ship. However this would still not warrant a stronger nacelle design as the hull it much weaker and would rip away from the ship first (as in the ST:VOY "Year of Hell" episode). The nacelles would probably be the last thing to go after explosive decompression on all decks, and by that stage, who cares.
When the shows talk about high warp causing a "shearing" effect to damage the nacelles on ships, including the Delta Flyer, it is also possible they are not referring to gravitational shearing forces, but simple aerodynamic drag. The deflector dish creates a field to push aside interstellar gas and dust which would damage the hull if it came into contact. The dish also has to scan ahead of the ship to identify any particles that the field is not able to move in time and target them specifically. This process has a limit too as the dish can only scan so far ahead of the ship, and the faster the ship is travelling, the less effective the deflector field will be. It is known that loss of the deflector dish would cause a ship to be ripped apart by insterstellar particles if it attempted warp speed. Therefore the same would be true if the ship exceeded the maximum speed where the deflector dish was effective.
I could go on and talk about the Bussard collectors on the front of each nacelle and how the field they produce to collect hydrogen could also affect the nacelles at high warp, but I'm hoping the crews know to switch off that field if it were causing a problem, lol.
Nerd Alert Over!
I'm really sorry again, I'll burn the book, and I swear I'll seek professional help.