I think its one of those things that leaves for for debate, it appears they dont quite stick to what Prometheus had shown, it appears the ideas had changed even in Prometheus by the time we got the movie Edit... which seems to indicate that only the Hammerpedes had Acid Blood, yet earlier Drafts we see Fifield has Acid Blood, and it appears the Engineers Face Burn was likely from Acid Blood of the Trilobite but this was changed with the Edits and Re-Shoots to be from the Juggernaught Crash.
When looking at Alien Covenant, it appears those Urns that was dropped had either been Primed in a Different way so they was activated to explode and in effect the Weapons Contents had a different effect compared to contamination from a NON-Weaponized/Activated Urn. I have before mentioned the Atomic Bomb Analogy. Were the Urns have a number of components and so LV-223 Outbreaks and Infections are like being effected by the Radiation/Leak from Uranium and Plutonium Material. Where as Davids Bombardment was like when a Nuke Explodes and the Uranium and Plutonium are used as Fuel to Make the Powerful Explosion that Incinerates those in the Blast Radius.
Where those out of the Blast Radius suffer instead the Radioactive effects of the Uranium and Plutonium. The other explanation is Davids Urns hit a Force Field that caused them to Explode and have a different effect, or indeed this was one Protective Element the other maybe after the Infection some Weapon was set off to Eradicate the Infection by unleashing a Burst of Energy that Burnt those Engineers and prevented any Mutations.
The way i look at the Neomorph Spores is that when the Juggernaught Crashed, some UN-activated Urns had came loose and Spilled the Black Goo to infect the Water Supply that was running down the Mountain were the Juggernaught Crashed, this infected the Water, which in turn infected Spores Growing from Mold near by Water Supplies... Fungi/Spores are not Plant Life but a type or Organism so they can be infected by the Black Goo.
I would say we would only had seen Hammerpedes if any natural Worm like Organisms got infected with the Black Goo Directly from a Leaked Urn and not Exploding one. It appears various life would have been infected by the Black Goo via leaking Urns in the Crashed Juggernaught and maybe from effects of Particles from the Bombardment that escaped the Blast Radius.
Holloway Question is a interesting one, which has a number of answers we could look at, especially if we look at the unused Props for his infection..... We can assume the Back Burster etc is result of infected Fungi/Spores. It appears Holloway was infected and he would have had tiny Parasitic Worms Multiply inside his body and they would begin to consume his Genetic Material overtime.
