Mutually assured destruction is a hell of a deterrent!

That's true. I don't think the reaction of the other factions toward Planet buster use is that irrational, considering the size of the craters they can make when detonated.

I even hated myself a little the first time I used a planet buster. I didn't know how destructive it would be... and I lost one of my own cities in the process. :(

Another thing that was not mentioned is that the AI tries to align itself into two blocks of factions competing against one another, lead by the two most powerful factions each. Due to this, even long term allies can become untrustworthy despite you treating them fairly throughout the entire game some time in the late mid-game.
It makes sense in trying to stop whoever is the most powerful (likely the human player) from winning the game, but feels sort of strange when for instance more trustworthy leaders like Lal or Deidre have been valuable allies since day one and align with your social values, but still suddenly shift drastically. If and when that happens, you should not try to hang on to the pact or whatever treaty you have at that point, since they will not stop undermining you no matter how nice and forgiving you might be. Have often seen it start with them sending probe teams over unless you are immune, which gives you the option to opt out of the pact or letting it slide. But if you choose the latter, the same probe team will just be back next turn over and over again.

Good to know! It explains some backstabbing I've seen in the past. I will keep it in mind next time I play the game.