It is interesting to look at the original Python build on edsy and compare to a pure G5 engineered version.
Guardian -
https://edsy.org/s/vD52Wff
G5 engineered -
https://edsy.org/s/vWsTKEJ
The Guardian version has 26.5% caustic resistance, 4509 raw integrity and 6135 caustic integrity.
The engineered version has 0% caustic resistance, 5970 raw integrity and 5970 caustic integrity.
As the build has no shields (phasing damage is apparently raw rather than caustic,
https://wiki.antixenoinitiative.com/en/thargoid-specs) then all Thargoid damage taken is caustic, so the Guardian build is nearly 3% stronger. Obviously you also have the other points mentioned elsewhere to consider like power usage, and ease of engineering vs getting Guardian HRPs, along with the much lower raw armour integrity.
However if you take the G5 build and swap out the engineered 2D HRP with a 2D Guardian HRP you end up with 5% caustic resistance, 5849 raw and 6156 caustic.
Or take the G5 build and swap out both the engineered 2D and 4D HRPs with a 2D and 4D Guardian HRP you end up with 9.8% caustic resistance, 5637 raw and 6246 caustic,
https://edsy.org/s/vvFLnJ2
This is why the general consensus is to use 1-3 Guardian HRPs, but if in doubt look at it in edsy first, then test in real life
