In a nucleus whose N/Z ratio is too large, the Pauli exclusion principle forces many of the neutrons to be in states with high energies. This makes the system less stable. For a fixed N, adding protons also makes such highly neutron-rich systems more stable, because the interaction between the protons and the neutrons is attractive, and the protons can go into low-energy states.
There is no Coulomb barrier for neutrons, so if a neutron has a high enough energy to escape, it just does -- no tunneling is required. Even if the system is bound, the system undergoes beta decay toward the line of stability.
There are nevertheless some systems with very high N/Z that are stable against neutron emission. E.g., 8He is bound and has a half-life of 119 ms.
The two pure-neutron systems that theorists predict might have the best chances of holding together are N=2 (the dineutron) and N=4. Experimental searches for dineutrons over a period of decades have failed to find any, so we're pretty sure they're unbound. These people claimed to have detected the N=4 system, with a lifetime of at least ~100 ns, which means it would have to be bound, although not stable with respect to beta decay. Whether they're right is a whole different question. I wouldn't bet a six-pack on it.