  The discovery of pulsations in several Ultraluminous X-ray sources (ULXs)
demonstrated that a fraction of ULXs are powered by super-Eddington accretion
onto neutron stars (NSs). This opened the debate as to what is the NS to black
hole (BH) ratio within the ULX population and what physical mechanism allows
ULXs to reach luminosities well in excess of their Eddington luminosity: strong
magnetic fields or rather strong outflows that collimate the emission towards
the observer. To distinguish between supercritically accreting BHs, weakly or
strongly magnetised NSs, we study the long-term X-ray spectral evolution of a
sample of 17 ULXs, 6 of which are known to host NSs. We combine archival data
from \chandra, \xmm\ and \nustar\ observatories to sample a wide range of
spectral states for each source and track each source's evolution in a
hardness-luminosity diagram (HLD). We find NS-ULXs to be among the hardest
sources in our sample with highly variable high-energy emission. On this basis,
we identify M81 X-6 as a strong NS-ULX candidate, whose variability is shown to
be akin to that seen in NGC 1313 X-2. Most softer sources with unknown accretor
show the presence of three markedly different spectral states that we interpret
invoking changes in the mass-accretion rate and obscuration by the
supercritical wind/funnel structure. Finally, we report on a lack of
variability at high-energies ($\gtrsim$ 10 keV) in NGC 1313 X-1 and Holmberg IX
X-1, which we argue may offer means to differentiate BH from NS-ULXs. We argue
that the hardest sources in our sample might harbour strongly magnetised NSs,
while softer sources may be explained by weakly magnetised NSs or BHs, in which
the presence of outflows naturally explains their softer spectra through
Compton down-scattering, their spectral transitions and the dilution of the
pulsed-emission, should some of these sources contain NSs.
