Just because you commonly see an item doesn't mean that it is undercosted. Nor does it mean that it is super effective.
If an item is something that is bought as a no-brainer for just about every army, yes it probably means it could be more expensive.
Armies that do not invest in the crown
TK - unbreakable
Vamps - unbreakable
Daemons - unbreakable
Dwarfs - not available
Wood Elves - no unit worth putting it on
I would say Daemons are in the "no available" rather than "unbreakable" category. I would give my eye teeth to have the Stubborn Crown on my Keeper of Secrets. Yes they don't have it either way but the point is it would be a 1+ item for daemons too.
I'll give you Wood Elves as less likely to have it but certainly I've seen Wood Elf armies with the Stubborn Crown.
Armies that don't need it but may take it
Empire - priest can give unbreakable + tons of ranks, GS are naturally stubborn, and captians/generals give cold blooded
have it. I really don't think I've seen an Empire army at a tournament without it. With the new book I expect it to be even more of a no-brainer - even though Empire have tons of Stubborn units, I would expect one large unit of halberdiers to have it or if a large core unit of knights is used, for them to have it. Particularly now it passes on to the detachments, Empire will always have a crown.
Skaven - almost always steadfast
Not units designed to actually fight. If we're talking units of plague monks, storm vermin or similar, a stubborn crown is mandatory unless they bring a bell or a furnace. Only the swarm of slaves build really can rely on steadfast as a tarpit.
Goblins - Almost always steadfast
Rarely, unless people are bringing the 100+ units of goblins.
Warriors - multiple means of being stubborn, rarely loose combats for reasons other than charging into a poor match up
Unless the build is designed around the Stubborn banner or Chosenstar shenanigans, yes, the stubborn crown is a must. Chaos players that assume their units are going to win no matter what don't tend to get far.
Ogres - default spell of their base lore, win most combats that aren't poor match ups (ie don't charge 12 bulls into 30 witch elves) and those combats that are lost this way the crown will at best buy you 1 more turn of turtling in on yourself before dying
Well, matter of opinion really. I consider it a 1+ item and I would never make a list without it. I am not going to rely on 1) winning combat or 2) getting a critical spell off.
Dark Elves - Because the stubborn peg lord is so powerful there is practically no reason not to use him. That said outside of that one character they don't need this item.
Of course they do! It's possible for mindrazor to get stopped once in a while.
Lizards - hard to kill units especially with slann support means they don't often give up combat res and any minor loses are going to be of minimum effect due to cold blooded. There is an arguement for it on a unit that can't be supported by magic or 'just in case' but it's far from needed.
The fact that their most important unit with general/bsb in it gets stubborn as standard plus a lack of fighty characters used, yes, I'd say Lizards rarely use this item.
Out of this list, I would say we have
Armies that need and make use of the crown universally
Orcs - different from goblins in that they will not always be steadfast
Beastmen - little/no armor means that faster races have a big advantage in tilting combat agaisnt them
Bretts - while they don't loose combat often and NEVER want to be stuck in, if they whiff their charge round they struggle to generate CR so may end up loosing due to large flanks and lack of attack power.
High Elves - not particularly good at CC and expensive troops means that they can always make good use out of the crown.
Warriors - other than chosenstar build (which is often comped out), will typically have the crown
Dark Elves - yes, a stubborn peg OR just in the main line
Empire - always
Ogres - I've not seen them without it
Most competitive Skaven - not seeing the unbreakable blocks as much these days.
That makes most armies and note that those armies that you are dismissing don't mean the item isn't too good. Daemons would LOVE the stubborn crown and probably dwarfs too. While dwarfs have multiple access to stubborn, they would jump on a 35 point rune to make another unit stubborn!
Basically, all you've got to say for armies that aren't going to have a huge incentive to take the item are:
Undead have no real reason (although useful for combat reforms!)
Some Skaven, Wood Elves and Orc and Goblin builds (but most competitive builds still take it)
Daemons and Dwarfs can't take it (but would if they could).
I'd say thats appropriate for a 35 pt item.
Most armies would pay much more than 35 points for the item and still consider it a 1+ item. Hell, I'd shell out 70 points in a beastman army for their version of stubborn.
Would you argue that the dispel scroll is OP because EVERY ARMY takes it?
I would say that it's undercosted, yeah. If an item is a 1+ regardless of circumstance, it's probably too cheap. I'd say the BSB is massively undercosted at 25 points these days. But since every army has it at the same price, it balances out. It isn't massively unreasonable that most armies would have a BSB as standard and since everyone gets the same value (minus undead, who often do without), it doesn't seem unfair to everyone. It also is worth an additional 100 victory points, which is much closer to the actual value of the BSB.
Just because something is undercosted, doesn't make it unfair. Just because something is unbalanced, doesn't make it undercosted. The hellheart seems about right in points - a good item that we ogre players often take, but not necessarily in every list and with points tight, this will get cut first. But its hideously unbalanced and can basically auto-win a game or do flat nothing.
The crown of command, however, is sufficiently cheap to be an auto-include for most armies and able to turn a certain defeat into a win on a regular basis, really a safety net for deathstars and fundamentally erodes the basic principles of tactics in the game into "kill the crownbearer or destroy the unit".