If only nuclear physics were that advanced! Each of the decay mechanisms are very different processes with separate theoretical treatments. I think it's fair to say that most decay rates can be reproduced by theory to within an order of magnitude or thereabouts. All but the lightest nuclei represent extremely complex many-body problems that cannot be solved exactly (not at the moment anyway). You can make general comments, like for a particular isotope (i.e. constant proton number) the beta decay half-life gradually shortens as you move away from the line of stability (increasing or decreasing the number of neutrons).