I don't think you should be able to program undead like a computer. Yes you can tell them to do a few things, but if every one of their actions is just on a clause then you can write up pages of coding for them and basically have them act as machines with complex programming. For example:
A reasonable command) "Attack any creature that enters this room that isn't me." Easy to follow.
An unreasonable command) "Attack any creature that enters this room that isn't me. If it's a group of people, aim for the ones that aren't wearing as much armor. If one of them falls then make sure you slit their throat. If more than half of you fall retreat and regroup to attack again. If most of you fall come and notify me."
They're not programmable machines, you can't enter an infinite set of "if-then" statements into them like one. If a DM ran them like this I would chuckle and start writing down pages of programming for them.
I suppose it largely comes down to how you define the coup de grace in the first place. If your table depicts the action as going deliberately for a vital point then mindless creatures are probably unable to do it. But if you imagine the action as basically just attacking a completely vulnerable enemy then there's no reason any creature couldn't do it.
One thing I might note is that it is generally assumed with skeletons at least that they can do some things outside of strict instructions to achieve their orders. Like if someone the skeleton was ordered to kill ran and slammed a door behind them, the skeleton would smash or open the door rather than just walk into it because it wasn't ordered to open the door.
I don't think you should be able to program undead like a computer. Yes you can tell them to do a few things, but if every one of their actions is just on a clause then you can write up pages of coding for them and basically have them act as machines with complex programming. For example:
A reasonable command) "Attack any creature that enters this room that isn't me." Easy to follow.
An unreasonable command) "Attack any creature that enters this room that isn't me. If it's a group of people, aim for the ones that aren't wearing as much armor. If one of them falls then make sure you slit their throat. If more than half of you fall retreat and regroup to attack again. If most of you fall come and notify me."
They're not programmable machines, you can't enter an infinite set of "if-then" statements into them like one. If a DM ran them like this I would chuckle and start writing down pages of programming for them.
I literally said that.
We keep track of the commands. Sometimes a page of text.
:D
But my group is weird, we find stuff like that fun, especially if it backfires and they wind up attacking each other (I had to paint numbers on my skeletons at one point to fix THAT glitch...)