Although the data are meager at present, particularly for exclusive semileptonic modes like B -> (pi) e (nu), the biggest uncertainty in the measurement of this parameter comes from disagreement among the various theoretical models. So sorting out the best approach is an urgent task.
Moreover, these approaches differ even more widely when it comes to the functional shape of the form factor. Since it will eventually be possible to measure the full weak form factor over a large range of momenta, this too needs to be reconciled.
Finally, there the purely theoretical challenge of tying parts of these results together. Generally the authors combine a solid calculation of a hadronic matrix element at a specific $q^2$ with a (motivated) extrapolation of form-factor shape. A successful approach to determining the form factor shape should be able to show that all these different $q^2$ determinations lie on that same curve.