  We present model-marginalized limits on mixed hot dark matter scenarios,
which consider both thermal neutrinos and thermal QCD axions. A novel aspect of
our analyses is the inclusion of small-scale Cosmic Microwave Background (CMB)
observations from the Atacama Cosmology Telescope (ACT) and the South Pole
Telescope (SPT), together with those from the Planck satellite and Baryon
Acoustic Oscillation (BAO) data. After marginalizing over a number of
well-motivated non-minimal background cosmologies, the tightest $95\%$ CL upper
bound we obtain is $0.21$ eV, both for $\sum m_\nu$ and $m_{\rm a}$, from the
combination of ACT, Planck and BAO measurements. Restricting the analyses to
the standard $\Lambda$CDM picture, we find $\sum m_\nu<0.16$ eV and $m_{\rm
a}<0.18$ eV, both at $95\%$ CL. Interestingly, the best background cosmology is
never found within the minimal $\Lambda$CDM plus hot relics, regardless of the
data sets exploited in the analyses. The combination of Planck with either BAO,
SPT or ACT prefers a universe with a non-zero value of the running in the
primordial power spectrum with strong evidence. Small-scale CMB probes, both
alone and combined with BAO, either prefer, with substantial evidence, non-flat
universes (as in the case of SPT) or a model with a time varying dark energy
component (as in the case of ACT).
