I recently obtained a book called "DEATHTRAPS", written by a WW2 vet who serviced the Shermans. I haven't read it, but in general he seems to say it's original cannon was not powerful enough, and that improvements were made over time.
The German tanks were well armored and defeated early Sherman attacks fairly well, except when whacked in the butt. But as frightening as those huge armored Panzers were, they were problematic as well. They were often maintainance nightmares, bogged down in muddy areas that our tanks maneuvered in pretty well, and often could not get through narrower town streets when they were designed for pedestrian or horse traffic.
Remember also, WE had to transport our tanks across an ocean .... the Germans had no such constrictions.