Non-Magic Immunity Sucks
Pretty much the title. I don't use it.
It kills player decision making as an influential thing and essentially level locks areas from the players which is poor design.
Running players up a tree to force them to make hard decisions can be good.
Caverns of Thracia does this at one point with a 1 way doorway trap into a room with sleeping gas and a singular wight. All other exits are locked. The wight has the key and kills with a touch.
This is a trap room and it is ok to design it this way until you see a single line in the wight's stat block. Immune to non-magical weapons.
This has instantly been turned into a no save or die situation to end up in. You just die unless you have passed a level test of have a magical weapon or have enough spells to burn this thing down (it has 20 HP so an average 2.5 hp on a magic missile will take like 8 shots to kill).
Congrats 90% of 1st level parties just TPKed to this thing no saves.
Fortunately for us we had a single magic weapon in the party which once we got the sleeping party member up that had it this turned into a trivial fight. 3 rounds of avoiding getting touched or just toss more bodies at it and you almost always win.
This is bad design. This is 100% a point I would look at it as a GM and redesign this encounter.
People proposed things like pull the wall hangings down and wrap up the wight. This is ineffective because it will eventually escape. You can't remove it as a threat. It felt like there was not a way to ever beat this encounter if you ever trigger it. The way you trigger it is by walking into it. It is guaranteed to eat at least 1 character.
People respond to my criticism of this with "this game was designed so you would lose characters but come back with a better plan" well guess what. Your PC cannot escape to share the information. They are magically trapped away from the party. To get information about the trap to beat it you must die and thus lock it away as meta knowledge.
I am not one to shy away from losing PCs. I will sit on the throne fully knowing there are magical runes there and better then even odds it is just a trap that will kill my PC.
I like the way Wolves does supernatural HD.
Change this encounter from
3HD 20HP ignore damage from non-magical weapons
to
3HD 20HP ignore damage rolls of under 6.
and keep everything else the same and this room turns into one full of interesting decisions as the party tries to get advantages to get those max damage rolls as efficiently as possible because 1 PC is dying basically every round.
Now you wrap it in wall hangings or pin it under pews and you set fire to it. So long as it is trapped in the fire it has a 1:6 chance of taking real damage. You can beat this thing with good planning and coordination.
If the very first plan fails there should still be a way at a cost to get your way out of a situation. Even if that is losing some characters or items.
This is much more enjoyable game play to me then "have this item or die."