Depending on who has Strident Spawn in play, Half-Orcs will be playable for one and not for the other.
This interpretation is result of this assumption:
I'm not contesting that distinction.
I'm contesting the statement that cards cannot be playable by one and not another.
How about Beornings?
"Playable at Beorn's House if the influence check is greater than 7."
Does it mean that before roll is made Beornings are not playable (and how to make influence attempt against them if they are not playable at this moment)?
How to explain phrase "Playable by a non-Ringwraith character at The Under-Gates" (The Balrog - ally)?
Ringwraith cannot move to The Under-Gates. Ok, he may be imprisoned in The Under-Gates, during movement to adjecent site (and "rescued" by Voices of Malice). Was phrase "by a non-Ringwraith character" added for such occasion?
My assumption is different. Phrases "playable" and "may be played" have identical common sense, similarly "remove from play" and "remove from game". For this reason sometimes they are used interchangeably.
And it is problem, because technically they are different terms.
Strider is removed from game (by its correct text), not from play, because otherwise we will have two manifestation of Strider - one in active play, second in Out-Of-Play pile.
However according to the Balrog Rules a manifestation of Balrog owned by Balrog player's opponent may be removed from play, not from game. Effectivelly preventing Balrog player from playing its avatar (if it would be correct).
For some reasons Dragon Rules says:
"If you defeat a Dragon manifestation that you played, it is
removed from the game and no one receives its marshalling points."
and
"• All existing manifestations in play of the same Dragon are removed from the game."
Sorry for this divagation about "from play"/"from play" problem. It was to illustrate of wider problem of wording. Where two terms that have identical common sense but different technical meaning are used interchangeably.
According to the assumption I'm taken "even if it may be played only by one, it is still playable generally":
Beorning are playable at Beorn's House and are successfully played if the influence check is greater than 7.
Half-Orcs are playable at protected Wizardhaven. May be played at tapped or untapped site if player has A Strident Spawn in play (please note "tapped or untapped" - you can influence an opponent's faction at the site where the faction is playable. Tapped status of site does not make the faction not playable even if it has not "if tapped or untapped" phrase in its text).
The Balrog is playable at The Under-Gates. May not be played by Rigwraith (please note that allies may also be played by Ringwraith oustside site where they are playable - as result of successful influence attempt).