The stability at constant volume for any system of elements can be described by its potential energy surface. In the cartoon below, the local minimum is a metastable state: it is higher in energy than the global minimum (the stable or ground state), but there is an energy barrier between the two, which is essentially caused by the need to rearrange the atoms in the system, breaking bonds. This is what prevents a metastable state from sponaneously transitioning to the ground state.
This is just an illustration; the real energy surface is a function of all atomic coordinates, and would require a high dimensional graph (3N dimensions for an N-atom system).
I don't think there is a satisfying answer to your question, since it is very much dependent on the system and conditions. Diamond (carbon), for example, is metastable at atmospheric pressure, but is the hardest known substance. This can be explained by the large energy barrier separating the diamond atomic configuration from the graphite (the stable state) atomic configuration, the strong bonds in diamond that remain strong at ambient pressure, the efficient packing of atoms, the hybridised atomic orbitals that give make the carbon atoms tetrahedrally coordinated, and so forth.
Hydrogen is a very different case. There is a very small energy difference between the different atomic configurations, and there are very many possible atomic configuration depending on the temperature and pressure (see for example http://www.nature.com/articles/ncomms8794). Hydrogen has a high zero-point energy, which can also affect the stability of these states, and at finite temperatures, there is also a free energy contribution to consider --- if you give the system some thermal energy, it can be pushed out of one minimum and into another.
Metallic hydrogen gets its exotic properties from the high pressure at which it is formed. The necessary electronic states require high pressure, such that the atoms are squeezed together. For most materials, when the pressure is reduced, and the atomic lattice is allowed to relax (non-constant volume), the electronic state will change, or the atomic structure will change due to vibrational effects (which are large for hydrogen). My suspicion is that if it is metastable, there is a low barrier to the ground state, and it will decompose easily.