Revan was powerful with (pre-capture) and without (Endar Spire/Taris) the Force. Thrawn learned about a species battle strategies by studying their art. Revan would create a new strategy that surpassed all Thrawn could ever know or learn. Revan could also use HK-47, the HK-50's and the HK-51's, who could be produced infinitely. If Revan simply bombed the Nohgri homeworld then Thrawn would be left with very few bodyguards. Imagine this:
An army of Stormtroopers, a handful of Nohgri Commandos, a few slaves, a Dark Jedi, and a fleet of Star Destroyers (Thrawn) against an army of Sith Troopers, Star Forge Droids, HK Assassin droids, Mercenaries, hundreds if not thousands of Sith and Dark Jedi, a Sith Lord, and a massive fleet of Ships constructed by the Star Forge and infused with the Dark Side (Revan). See how it balances out?
Thrawn was a strategist, but Revan was a strategist and a leader. Revans leadership ability and charisma could keep his soldiers loyal to him to the death. Many beings only allied themselves to Thrawn for personal gain. Revan's assassins could also capture and corrupt Thrawn's forces, adding to Revan's might.