My point is, that both players and GMs in a game need to think "Are we OK if the antagonists can use this really powerful thing, as well as the protagonsits?" before applying such rules.
In my experience, players who suggest things like this are the loudest and quickest to complain when the GM uses the same rules against them.
So, whenver a player suggests a rule change like this, I make sure to explicitly ask them 'Are you OK when this happens to your character?"
I really dont get how DMs cant run it via RAW and RAI:
Player: I sneak up the guards.
DM: Cool. Roll steath mate.
Player: (rolls) An 18!
DM: (checks passive perception) The two guards sits there quietly gazing off into the distance whistling to themselves as you sneak up. You're now 30' away.
Player: I shoot the first guard in the face with my crossbow!
The player has now declared an action. and the DM has determined that it is time to switch to combat sequence.
DM: OK mate, you lift your crossbow, take aim and fire! Roll initiative. The guards rolled a 13.
Player: (rolls) Damn, a 10.
DM: (thinks a second). OK, the guards go first. Hearing the twang of your bow at the last possible second, the guards stops whistling; before they can turn around you can resolve your attack. Theyre both surprised and cant do anything other than stand there like idiots on their turn. Its now your turn, lets resolve that crossbow attack.
Player: Do I get advantage from assasinate?
DM: Yes; im happy to rule that the guards havent yet 'taken' a turn this combat. However, as the guards are no longer surprised (having beaten you via initiative), your attack is not an automatic critical hit. You're hidden anyway, so you still get sneak attack to this attack, and advantage.
Player: Cool (rolls to hit, rolls damage).
DM: The first guard cops it in the back of the skull and staggers forwards in stunned silence, a soft gargle of blood coming from his mouth, possibly calling out the name of a sweetheart - her name the last word that ever passes his lips.
Player: It says 'Neutral Evil' in the alignment section of my character sheet for a reason DM. I'll use cunning action to hide.
DM: No-one ever thinks of the henchmen... I'll allow the hide check. The second guard is still in shock with what is happening, and hasnt yet had a chance to turn around and thus cant see you 'clearly enough'. What is your stealth check result?
Player: (rolls) 15...
And so forth.
I've never understood why some DMs find this so hard to wrap their heads around or narrate.