Sure, Valkyrie's power is "underwhelming" to some, but the essence of it is that he takes damage and converts it to strength and size... which is really powerful as some others have said. It's a "miracle" in terms of healing and powering up.
This fits the SS's theme well: Lille was God's (Yhwach's) Messenger, and Valkyrie is God's "Miracle", one who was blessed with this ability and is the "enforcer" of God's will; a literal "physical embodiment" of it.
The way I see it, there have to be a couple of "unconventional" ways to stop Valkyrie, since it seems like Brute Force will do nothing (which, unlike some have said, makes Kenpachi the WORST matchup for Valkyrie, since Kenpachi is power personified... Valkyrie will just take his wounds and make himself larger and stronger):
- The Captains figure a way to stop the Miracle power from occurring (This may fit Shinji's Bankai: Remember how his Shikai inverted senses? WHAT IF HIS BANKAI INVERTS ABILITIES? *hint hint*)
- They completely obliterate his being (from what we saw, Byakuya did not completely vaporize Valkyrie, leaving his lower half. Does the "Miracle" perhaps depend on having a small part of your body to be able to regenerate?)
- Bankai: Plot. Self-explanatory.