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Methods. We analyzed broadband XMM-Newton and NuSTAR observations of NGC 300ULX1, performing phase-averaged and phase-resolved spectroscopy. We comparedtwo physically motivated models for the source spectrum: Non-thermal accretioncolumn emission modeled by a power law with high-energy exponential roll-off(AC model) vs multicolor thermal emission from an optically thick accretionenvelope plus a hard power-law tail (MCAE model). We combine the findings ofour Bayesian analysis with qualitative physical considerations to evaluate thesuitability of each model.  Results. The low-energy part (<2 keV) of the source spectrum is dominated bynon-pulsating, multicolor thermal emission. The (pulsating) high energycontinuum is more ambiguous. If modelled with the AC model a residual structureis detected, that can be modeled using a broad Gaussian absorption linecentered at ~12 keV. However, the same residuals can be successfully modeledusing the MCAE model, without the need for the absorption-like feature. Modelcomparison, using the Bayesian approach strongly indicates that the MCAE model-- without the absorption line -- is the preferred model.  Conclusions. All models considered strongly indicate the presence of anaccretion disk truncated at a large distance from the central object, as hasbeen recently suggested for a large fraction of both pulsating andnon-pulsating ULXs. The hard, pulsed emission is not described by a smoothspectral continuum. If modelled by a broad Gaussian absorption line, the fitresiduals can be interpreted as a cyclotron scattering feature (CRSF)compatible with a ~10^12 G magnetic field. However, the MCAE model describesthe spectral and temporal characteristics of the source emission, without theneed for an absorption feature and yields physically meaningful parametervalues. Therefore strong doubts are cast on the presence of a CRSF in NGC 300ULX1.

Investigating ULX accretion flows and cyclotron resonance in NGC 300 ULX1