Their defense looks pretty good, especially run defense. Georgia Tech is pretty inept passing, and if Miami shuts down the run Tech will punting all day. GT ranks near dead last in a few passing stats nationally, and Miami stuffs the run.
#7 in the nation in rush defense. Only allow 2.6 yards per carry as well. They allow 77 rush yards per game. #20 in scoring defense... They gave up 42 to Virginia Tech, but that was really more of a product of having 5 turnovers in the game. They threw 4 interceptions on 4 straight possessions to give the game away.
The reason Vegas has the spread at 20 is because they don't believe Georgia Tech will score, or at least not score more than 3-10 points.