Context. The B/C ratio is used in cosmic-ray physics to constrain the transport parameters. However, from the same set of data, the various published values show a puzzling large scatter of these parameters. Aims. We investigate the impact of using different inputs (gas density and hydrogen fraction in the Galactic disc, source spectral shape, low-energy dependence of the diffusion coefficient, and nuclear fragmentation cross-sections) on the best-fit values of the transport parameters. We quantify the systematics produced when varying these inputs, and compare them to statistical uncertainties. We discuss the consequences for the slope of the diffusion coefficient delta. Methods. The analysis relies on the propagation code USINE interfaced with the Minuit minimisation routines. Results. We find the typical systematic uncertainties to be greater than the statistical ones. The several published values of delta (from 0.3 to 0.8) can be recovered when varying the low-energy shape of the diffusion coefficient and the convective wind strength. Models including a convective wind are characterised by delta greater than or similar to 0.6, which cannot be reconciled with the expected theoretical values (1/3 and 1/2). However, from a statistical point of view (chi(2) analysis), models with both reacceleration and convection - hence large delta - are favoured. The next favoured models in line yield delta, which can be accommodated with 1/3 and 1/2, but require a strong upturn of the diffusion coefficient at low energy (and no convection). Conclusions. To date, using the best statistical tools, the transport parameter determination is still plagued by many unknowns at low energy (similar to GeV/n). To disentangle all these configurations, measurements of the B/C ratio at TeV/n energies and/or combination with other secondary-to-primary ratios is necessary.