The Oni Titan, so far as I can tell, is considerably larger. Plus, magic tends to beat out technology in the Ninjago universe:
The Great Devourer trashed the Bounty and Ultra Sonic Raider.
The Preeminent proved more than a match for the Ninja’s vehicles.
Aspheera and Boreal both proved more than a match for the Titan Mech.
I do feel like the Titan Mech was probably built in-universe as a precaution against threats like the Colossus, but from what we saw I don’t think it would have fared well.
