As per the 2024 rules update (which I have beef with but am using here to make my point) :

Resurrection

Level 7 Necromancy (Bard, Cleric)

Casting Time: 1 hour Range: Touch Components: V, S, M (a diamond worth 1,000+ GP, which the spell consumes) Duration: Instantaneous

With a touch, you revive a dead creature that has been dead for no more than a century, didn’t die of old age, and wasn’t Undead when it died.

The creature returns to life with all its Hit Points. This spell also neutralizes any poisons that affected the creature at the time of death. This spell closes all mortal wounds and restores any missing body parts.

Coming back from the dead is an ordeal. The target takes a −4 penalty to D20 Tests. Every time the target finishes a Long Rest, the penalty is reduced by 1 until it becomes 0.

Casting this spell to revive a creature that has been dead for 365 days or longer taxes you. Until you finish a Long Rest, you can’t cast spells again, and you have Disadvantage on D20 Tests.

I cast Resurrection on the lich BBEG. In 5e Resurrection no longer states that the soul must be willing to return in order for it to work, and there's no save, so it should just work if I'm able to touch him. Takes an hour to cast but we're not worried about that right now.

Does it resurrect him properly? New mortal flesh, soul stuffed into it, meaning he is now no longer immortal and loses most of his legendary actions, and the phylactery becomes inert because it's no longer containing a soul? Extending from this, is a proper resurrection just a "get out of undeath free" card and if so why don't we see it used on every undead? It specifies and wasn’t Undead when it died but I think most Undead go from Living to Dead to Undead in that order, liches included.

Does it just instantly dust him, like throwing a Phoenix Down at an undead does in Final Fantasy?

This used to be a solved problem, but between 2014 and 2024 they changed the wording on Resurrection from

You touch a dead creature that has been dead for no more than a century, that didn't die of old age, and that isn't undead. If its soul is free and willing, the target returns to life with all its hit points.

to, now:

With a touch, you revive a dead creature that has been dead for no more than a century, didn’t die of old age, and wasn’t Undead when it died.

There must be a reason why this was changed. I need answers.