Why would you even want unconditional love? Are you planning on abusing and taking advantage of your spouse? People who want this are not ready for marriage.
Like privacy, people can come up with nice catch-alls like “nothing to hide”, but the default is not abuse.
and the same goes in reverse. i love my wife and only wish the best for her. but that does not mean i need to be blind to her problems, or not expect her to stop doing things that hurt me or others. but it does mean that my love for her does not stop even if we should separate.
however, this assumes a different kind of love than the love that most people think they have for their partner. a kind of love that is exclusive and can only ever go to one person. unconditional love is the kind of love that accepts everyone for what they are. the kind of love that all the major religions out there are talking about.
(edit: expanded further)
In the sense that there might be something you could say to tank a relationship? Sure. But that's a far cry not generally being able to express "unfinished opinions" or any vulnerability, as the original commenter said.
You can leave someone you still love, if they did not meet what you desire out of the relationship. That would still be unconditional love.
But with unconditional love, leaving someone is to heal yourself, not to punish the other person.