In case you haven't worked it out by now, I love writing about game theory as it applies to adventures. This comes from 40 years' experience playing, writing and writing about adventures. So please forgive me if you find this boring, but it's meant to be thought provoking.
Here's some more thoughts on the problems of an implicit OPEN after UNLOCK.
How many turns does it take to do an implicit open after unlock? One or two? With non-implicit unlock and open, it takes two turns, as there are two explicit commands resulting in two explicit actions and these actions can be separated in time. A common sequence would be UNLOCK DOOR WITH KEY. DROP KEY. OPEN DOOR. That's clearly 3 turns. With implicit actions, it should be the same, so you have to manually increment turns after the initial UNLOCK.
What happens if you attempt to UNLOCK DOOR when it's already unlocked? You can't unlock it again. Do you just give an error response or do you open the door, even though that's not what the player asked for?
From the above, it should be pretty obvious that you can't just make UNLOCK and OPEN synonyms, otherwise you would be able to say UNLOCK DOOR when it wasn't locked in the first place.
Earlier, I said that you must be consistent so as not to surprise the player with seemingly nonsensical responses. Therefore, if you have a locked box or a locked chest, these should also be automatically opened after an UNLOCK command if you have an implicit OPEN. But what happens if you don't want them automatically opened? Consider Pandora's box. You need to unlock it, but you don't want to open it or you'll unveil the horrors within. I recall a game like this where you had to unlock the box, but not open it and give the unlocked box to a monster. When the monster opened the box, the monster was killed when the contents of the box escaped. With your automatic OPEN after UNLOCK, it would be impossible to complete the game, because unlocking the box also opens it and the contents kill you.
Chris mentioned 'The Witch's Apprentice'. It contains a puzzle near the beginning where you need to unlock the front door, but you don't do it using a key. Are you suggesting that the unlock operation should also open the door, even when there's no one in the vicinity to do it? I don't think so.
What happens with LOCK and CLOSE? If you close the door, is it automatically locked? You were suggesting that it should be because that's what you would "naturally" do in the real world. But what if you've dropped the key somewhere? And what happens if you try to LOCK the door when it's open? You can't do this in real life. Are you suggesting that you should automatically close the door first, then lock it? Then we're back to the same problem the key is missing.
So, carefully consider ALL the consequences of using implicit OPEN after an UNLOCK.