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 \gtrsim 0.6 , which can not 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 ( \sim 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 .