@shadyJ will have more technical insight.
To me, my gut feeling says that speakers are speakers. Just because something is labeled “floor standing” does not mean you can’t put them inside a wall. The salient thing is fitting into the wall and as
@mazersteven says, proper placement of those speakers.
I would even say that floor standing speakers might be even better than many “in-wall speakers” because the floor standing speakers have better rear enclosures/back boxes than many in-wall speakers.
Is putting floor standing speakers inside a wall better than putting them into the room? Probably not because you can change the placement easier if they are free standing, instead of being stuck in one position inside a wall.
Is putting floor standing speakers inside a wall better than other typical in-wall speakers that might not have the best rear enclosures? Probably yes if the placement is good.
Will the speakers be flushed with the new wall like how a typical in-wall speaker would be?