How aerial combat in grim darkness of future looks?
Is having mechadendrites while not being in mechanicus techheretical?
Why would planetary governor even need geneseed, is it of any use to anyone except astartes/ chaos?
Promethium is used to make nutrient paste? How would you eat fucking oil, really?
Why salamanders did not use phospex?
Aerial combat varies depending on what factions are fighting. Imperial aircraft tend to be VTOL craft with limited mobility but armour that rivals battletanks, Eldar aircraft are supersonic but fragile, Ork ones are ramshackle and unpredictable like their ground vehicles, Chaos and Tyranids use lots of flying creatures that fight in a manner more akin to dragons than anything else.
Mechadendrites are powered by a set of complex augmetics grafted to the spine and torso cavity of tech priests called the Potentia Coil. They are one of the sacred augmetics restricted to tech priests. By extension only ordained members of the Mechanicus can have mechadendrites. Having a mechadendrite is not in itself heretical, but it is a sign that you have consorted with rogue tech priests to have it grafted to you since no member of the Mechanicus would give someone a Potentia Coil.
Planetary Governors don't have any use for geneseed. It requires rare knowledge to cultivate the organs implanted into marines from it and such knowledge is possessed by few outside the forces of Chaos or the Mechanicus. Some Chaos factions do eat it as a delicacy though, so some particularly decadent governors may do so as well. Being found in possession of geneseed would probably be justification for summary execution though.
As mentioned by Sheb promethium is a generic term for a wide variety of hydrocarbon compounds. Some are similar to crude oil, others are more like vegetable oil, some are napalm and some are space magic chemicals. Like modern uses for oil it can be synthesized into a lot of things, like plastics.
Phosphex is restricted technology and is borderline heretical. The Space Marine Legions used it in the Great Crusade and even then considered it a monstrous weapon. I don't have the Horus Heresy rulebooks so I don't know much about it, but it was considered one of the most destructive and cruel weapons available to the Legions. After the Horus Heresy and it's use of various weapons of mass destruction the Imperium forbid the use of many of the darker and more destructive technologies of the time for fear that they would lead more marines down the dark path of the Traitor Legions.