  Far-infrared (FIR) emission lines are a powerful tool to investigate the
properties of the interstellar medium, especially in high-redshift galaxies,
where ALMA observations have provided unprecedented information. Interpreting
such data with state-of-the-art cosmological simulations post-processed with
CLOUDY, has provided insights on the internal structure and gas dynamics of
these systems. However, no detailed investigation of the consistency and
uncertainties of this kind of analysis has been performed to date. Here, we
compare different approaches to estimate FIR line emission from
state-of-the-art cosmological simulations, either with CLOUDY or with
on-the-fly non-equilibrium chemistry. We find that [CII]$_{158\mu}$ predictions
are robust to the model variations we explored. [OI] emission lines, that
typically trace colder and denser gas relative to [CII]$_{158\mu}$, are instead
model-dependent, as these lines are strongly affected by the thermodynamic
state of the gas and non-equilibrium photoionisation effects. For the same
reasons, [OI] lines represent an excellent tool to constrain emission models,
hence future observations targeting these lines will be crucial.
