Neutron star matter works for spaceship hulls if you use the proper level of retaining force fields. These must be triple redundant, as accidents have occured with result in a white-hot sphere of neutronium a few yards in diameter.

You must also make the hull perfectly spherical to deal with the internal gravitational effects.

Usually, it's considered overkill to use this material for hulls. It makes you difficult to approach from outside without proper force field merging techniques, and the force field energy required to simply maintain a door or sensor penetrations is prohibitive. Mostly such hulls are considered overexpensive status symbols, although the mirror-smooth finish is quite attractive. The standard General Products hulls are considered better alternatives, particularly since they come pre-drilled.