and
3) He doesn't have FDR's leadership qualities. That is, I don't think he'll be able to reach out to the American people and restore their confidence. (Though it would be interesting to see how much FDR actually did that.)
4) He doesn't have FDR's connection to the Eastern Establishment and the Ivy League intellectuals..
At this time, New York is the absolute capital of American business. One American in eight lives in the New York metro area. FDR is a New Yorker, with a legion of friends and contacts at the highest levels there. He went to Groton, Harvard, and Columbia. Garner is a hick from deep-rural Texas, who dropped out of Vanderbilt. He has contacts in Texas and in Congress.
Garner won't have an FDR-style Brain Trust. Though as pointed out, he was a Wilson progressive of sorts, and Wilson had a proto-Brain Trust going during WW I
But in general I agree that Garner is perhaps too easily pigeonholed. The man often rises to meet the occasion.