Listen, I was a bouncer and trained (very lightly) in BJJ and grappling. Moreover, I knew and still know a lot of MMA-trained individuals. As a general rule, someone trained in an MMA discipline has the advantage over someone not trained in anything. But: 1) Just because someone trained and competed in MMA does not mean they are automatically good in a fight, and 2) someone not having training in such a discipline does not mean they are bad in a fight.
Size and explosive power go a long way, but many times it is the willingness to do what it takes to win a fight that makes a difference. Someone willing to break a finger; fish hook an eyeball; or crush a testicle have the upper hand on someone that has perfected how to transition into an omoplata from someone trying to ground and pound them in the mount. Can they end a fight quickly with as much as one strike? Obviously. But if they get their bell rung first does that mean their training will automatically overcome that?
Reigns is a former collegiate and professional Defensive Lineman that grew up in a family that obviously passed along the time honored tradition of grappling and being able to take care of themselves. He is not a male model with all show muscles that was promoted up to be the top scripted fighter. He has about 70 pounds or even more on Riddle. He has shown exceptional strength. To think Riddle wipes the floor with him is laughable.