I never really found the NX design to have been. Really well thought out. I mean, it's the beginning of the warp era for humanity, and the essentially stick a nuclear reactor right next to where the crew sleeps. I always figured the early Starfleet ships had been designed with a secondary hull (that was continued through the centuries) to keep the engine away from the crew so that if anything happened, there would be some time to get the saucer seperated and most of the crew away from the problem.
Maybe by the time of TNG/DS9/Voyager the designers had designed something into the bulkheads, and implemented force fields,to protect the crew so they could stick the warp core next to the sleeping cabins.