Why is it so difficult to create mock NMR spectra of macromolecules?
Macromolecular NMR spectra are difficult to compute because macromolecular electronic structures are computationally expensive to solve with accuracy. Obtaining precise chemical shifts and splitting requires accurate treatment of electron density and geometry, especially for simulation of NOESY, which I understand is important in macromolecular structural elucidation. This requires good methodology and large basis sets.
An interesting case study by Saielli and Bagno demonstrates the use of DFT (with cc-PVTZ basis) in predicting NMR spectra for two candidate structures and the correlating them with experimental data, however the mean error in chemical shifts is nontrivial1 (0.2 ppm in the best match). The same people do $^{1}\ce{H}$ and $\ce{^{13}C}$ NMR predictions of a number of complex molecules2 (with intriguing discrepancies in shifts) however this is a far cry from predicting, say, a large protein $^{1}\ce{H}$ NMR spectrum.
I have seen EPR spectra of transition metal clusters simulated as abstract spin systems (for instance by EasySpin) however I presume that this is very qualitative.

(1) Saielli, G.; Bagno, A.; Can two molecules have the same NMR spectrum? Hexacyclinol revisited.; Org. Lett.; (2009) 11, pp. 1409-1412
(2) Bagno, A.; Rastrelli, F.; Saielli, G.; Toward the complete prediction of the $\ce{^1H}$ and $\ce{^{13}C}$ NMR spectra of complex organic molecules by DFT methods. Application to natural substances.; Chem. Eur. J.; (2006) 12, pp. 5514-5525