Speed, speed speed. He hit "elite TE" thresholds for size-adjusted speed and 10 yard split. Offers a downfield dimension that Ferguson and Hendershot never will.
Willing to block. It's tough to play for Harbaugh otherwise.
Cons:
Way overaged prospect who will be a 25yo rookie this September.
For a 4 year player, his production is frankly trash. 54 career catches is only 7 more than Luke Musgrave, who caught a bunch of flak for being unproven.
His blocking won't translate any better than Schultz's did. He's not fundamentally very big or strong by NFL standards, and he was a lot older and more physically mature than a lot of the college guys he was facing. Schoonmaker's got the want-to, but he may not have the power to be a real factor as a blocker.
One additional pro is versatility. McCarthy specifically mentioned he had experience in all four area's where you could potentially play a TE (inline, slot, backfield, etc.).