Subsists is more ambiguous.
If by “more ambiguous” you mean less simplistic, more nuanced, and actually more precise…then yes. But that is a simplistic and ignorant way to look at it.
“Subsists in” means that the Catholic Church is the truly the institution in which the Church of Christ essentially and perfectly continued. But without excluding the obvious fact that there are valid sacraments in the Orthodox, and valid baptism in many Protestants.
It’s like a body that has lost limbs, let’s say. If the limbs falls off, they die, soon enough the life is no longer in them. And we would say that the body “subsists in” the living part, the part with the torso and head. But it would be less precise to say the torso and head “is” now the body, as there is not a perfect complete identity between the two concepts due to the damage. In a certain sense, the lost limbs “are” body too (because if they were reattached they would clearly be part of the life of the body), but the essence of body does not subsist in them like it does in the part that still has the heart, and brain, and soul.
Metaphysically it has a precise meaning.
First, you should realize that according to the metaphysical meaning, something can only SUBSIST IN** one** thing. Many things can theoretically “be” something. But in only one thing will it “subsist”. So the term still makes the Catholic Church unique. It was not intended to imply that it could then also subsist in other things, as that is metaphysical nonsense given the precise meaning of “subsist”… It was certainly not intended to convey something like, “The Catholic Church is A Church of Christ” as if to merely affirm that we were a valid Christian Church without excluding others. It was to affirm that we were THE valid Christian Church, without excluding the reality of valid Orders in the Orthodox and baptism among the protestants.
When we say, “the Catholic Church is the Church of Christ” it could be mistaken that, “Oh yeah, so is the Orthodox Church”. Like, “this is the headquarters of XCorp.” Well, they might have two headquarters, one in Europe or something. “Is” is a VERY ambiguous term.
It’s like telling a new person in town, “St. Gertrude’s is the Catholic Parish” or “Chicago is the Catholic Archdiocese”. But there can “be” OTHER Catholic parishes and Archdiocese in the world, and niether of these is essential to the Church. However Catholic “Parish-ness” SUBSISTS IN the Pope’s parish at St. John Lateran, and Catholic diocese-ness SUBSISTS IN the diocese of Rome, the essential “core” in which they subsist even if others can be united and thus be those things.