First, the whole Impale feature can only be performed as a standard action, which means it won't do as much damage as a full attack at higher levels
I mean, there's mechanics to add it to a charge, and a full attack often isn't feasible. It's not like you are always trading it for a full attack. And you can try every round.
Pinning the enemy in place also restricts your weapon - you're no longer threatening other spaces around you
Where is this restriction? I see one about not using the weapon to attack other creatures, but that's very different from no longer threatening. Also, it's not clear what happens if you just release the weapon- there's rules for dealing with a weapon that is impaling a creature that no one is holding, but there's other rules saying that the impalement can be ended if you are disarmed.
and the foe you've impaled can disarm you. (This is a viable strategy in Spheres of Might - the Duelist sphere focuses on disarming and is quite handy.)
Presumably you can find someone besides a guy stacked high in Duelist talents for your impale then. Certainly dropping this into a regular game would prove disruptive. Then you also have the rules quandary listed above.
I don't think "easy stunlock" is quite the right way to describe how the ability works in a game unless foes are very dumb and only ever attempt checks to get free
Certainly a typical group of PCs will have someone, and possibly many someones, against whom this is an easy stunlock- and the same is true of most encounters.
Oh, and Hold Person is a thing.
Hold Person is well understood by the REST of the game rules. In addition to the obvious type restriction, it is understood that increasing your will save is good against this entire class of ability, and plenty of things allies can do interact with this, such as effects that help will saving throws, effects that help paralyze, etc. Your allies can also dispel your Held state with a variety of spells and abilities. It's vastly more interactive than this, which requires houserules just to interact with the spell set at all, and doesn't have any built in hooks to be countered by the large variety of magical items, spells, racial abilities, and class abilities that already exist to prevent a character from being pinned down in this matter.
And again, hit points are abstract: being impaled is not abstract.
the books would be a lot thicker if every ability tried to cite every other effect that might work against it. XD
That's the level of effort you have to go through when you add something like this though.
Did you ever deal with 2nd edition psionics? It used the standard magic stuff as sort of a baseline, but it made two core mistakes which resulted in it being wildly overpowered. The first was, it used checks against your OWN abilities instead of checks against ENEMY abilities. This worked from a lore perspective, but it really meant that you were rolling against your own high constitution (or whatever), with small chance of failure, no matter whether you were going to mess with the mind of a dog or a dragon. The second core problem psionics had was that it was a parallel construction of magic. You'd have a "disintegrate" power that worked pretty much like the spell, complete with saving throw, but anything that worked to defend against magic didn't work against it. Meaning that, for instance, high levels of magic resistance offered no protection.
The error in this book isn't nearly as egregious, but it still uses a very fluid stat that is balanced around damage dealing (Armor Class) as a key for a control method. In Pathfinder, control methods use the CMB/CMD mechanics (aka, Impale not using CMD is a design flaw), and that's also where you go to find your defensive boosts against these things. By having rolls that have no meaningful chance of failure (attack rolls), forcing rolls that have no meaningful chance of success (concentration versus 1.5x level plus stat, forcing attacks against your CMD with no bonuses, even to an expert grappler / tripper / repositioner / dragger), it plays by rules that aren't interactive with the rest of the game.
Also I want to point out: I've ONLY looked at the Lancer part. I went there first because I know that lances are generally not rewarding enough for a player that wants to use them, and I was thinking, hey, maybe they fixed this here, maybe I could find something I could drag right on in to my games. Maybe I landed on the most disruptive thing I could find strictly by chance: maybe not.