Big signings are risky, and there's always the chance of catastrophic collapse in any player (look at Grady Sizemore, for example).
But it's worth mentioning that Betts is simply a different caliber of player than McCutchen (or Sizemore) at the same age.
Take a gander.
Whatever the merits/demerits of WAR, it does provide a rough framework for understanding a player's value.
Betts is just behind Bonds, Mays, and Musial through age 26 and a fair bit ahead of Tim Raines and Dick Allen (Sizemore and McCutchen are on the next page).