There are always a lot of unknowns
when a team has to fill a top coaching position. Two that I can think of right off the bat are the length of time it will take for the players to adopt and learn the new scheme and the availability of new players to fill the needed roles of the new defense.
The few positives that come to mind are Ryan's familiarity with the new style of defense and his friendship with the head coach. I say that the latter is a positive simply because they will work well together and they will each be frank about what is needed and expected regarding players and expectations without any 'power trips' or personality clashes.
But that's just MHO.