"Trustworthiness" of vertical transformations

Hi list,

I've seen a lot of activity on the PROJ repo lately relating to
vertical datum transformations. Coming from zero knowledge about the
world of vertical transformations... if I setup a simple
transformation between two proj CRS objects created using auth/id
codes, and transform a 3d point using proj_trans_generic... how
"trustworthy" is the resultant z value?

I've seen some examples (e.g in
https://github.com/OSGeo/PROJ/issues/1743) which use a
"EPSG:26911+5703" type construct, which leads me to believe that the
world of vertical transformations is not so straightforward! Any tips
to give a newbie entering this world to avoid the inevitable pitfalls
which await?

Cheers,
Nyall
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