  Carbon-enhanced metal-poor (CEMP) stars are a unique resource for Galactic
archaeology because they probe the properties of the First Stars, early
chemical evolution and binary interactions at very low metallicity. Comparing
the fractions and properties of CEMP stars in different Galactic environments
can provide us with unique insights into the formation and evolution of the
Milky Way halo and its building blocks. In this work, we investigate whether
directly comparing fractions of CEMP stars from different literature samples of
very metal-poor ([Fe/H] < -2.0) stars is valid. We compiled published CEMP
fractions and samples of Galactic halo stars from the past 25 years, and find
that they are not all consistent with each other. Focusing on giant stars, we
find significant differences between various surveys when comparing their
trends of [Fe/H] versus [C/Fe] and their distributions of CEMP stars. To test
the role of the analysis pipelines for low-resolution spectroscopic samples, we
re-analysed giant stars from various surveys with the SSPP and FERRE pipelines.
We found systematic differences in [C/Fe] of ~0.1-0.4 dex, partly independent
of degeneracies with the stellar atmospheric parameters. These systematics are
likely due to the different pipeline approaches, different assumptions in the
employed synthetic grids, and/or the comparison of different evolutionary
phases. We conclude that current biases in (the analysis of) very metal-poor
samples limit the conclusions one can draw from comparing different surveys. We
provide some recommendations and suggestions that will hopefully aid the
community to unlock the full potential of CEMP stars for Galactic archaeology.
