  Multiple codes are available to derive atmospheric parameters and individual
chemical abundances from high-resolution spectra of AFGKM stars. Almost all
spectroscopists have their own preferences regarding which code and method to
use. But the intrinsic differences between codes and methods lead to complex
systematics that depend on multiple variables such as the selected spectral
regions and the radiative transfer code used. I expand iSpec, a popular
open-source spectroscopic tool, to support the most well-known radiative
transfer codes and assess their similarities and biases when using multiple
set-ups based on the equivalent-width method and the synthetic spectral-fitting
technique (interpolating from a pre-computed grid of spectra or synthesizing
with interpolated model atmospheres). This work shows that systematic
differences on atmospheric parameters and abundances between most of the codes
can be reduced when using the same method and executing a careful spectral
feature selection. However, it may not be possible to ignore the remaining
differences, depending on the particular case and the required precision.
Regarding methods, equivalent-width-based and spectrum-fitting analyses exhibit
large differences that are caused by their intrinsic differences, which is
significant given the popularity of these two methods. The results help to
identify the key caveats of modern spectroscopy that all scientists should be
aware of before trusting their own results or being tempted to combine
atmospheric parameters and abundances from the literature.
