When combining cosmological and oscillations results to constrain the neutrino sector, the question of the propagation of systematic uncertainties is often raised. We address this issue in the context of the derivation of an upper bound on the sum of the neutrino masses ($Sigma m_ u$) with recent cosmological data. This work is performed within the ${{mathrm{Lambda{CDM}}}}$ model extended to $Sigma m_ u$, for which we advocate the use of three mass-degenerate neutrinos. We focus on the study of systematic uncertainties linked to the foregrounds modelling in CMB data analysis, and on the impact of the present knowledge of the reionisation optical depth. This is done through the use of different likelihoods built from Planck data. Limits on $Sigma m_ u$ are derived with various combinations of data, including the latest Baryon Acoustic Oscillations (BAO) and Type Ia Supernovae (SN) results. We also discuss the impact of the preference for current CMB data for amplitudes of the gravitational lensing distortions higher than expected within the ${{mathrm{Lambda{CDM}}}}$ model, and add the Planck CMB lensing. We then derive a robust upper limit: $Sigma m_ u< 0.17hbox{ eV at }95% hbox{CL}$, including 0.01 eV of foreground systematics. We also discuss the neutrino mass repartition and show that todays data do not allow one to disentangle normal from inverted hierarchy. The impact on the other cosmological parameters is also reported, for different assumptions on the neutrino mass repartition, and different high and low multipole CMB likelihoods.
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