Several of the division have very little turnover among the top 3. You can certainly make an argument for Large Senior being among the most difficult to win divisions, but, in fact, Large Senior was the only club division won by a team that didn't medal the year before. I'm not suggesting that this means LS is an easy division, just saying that you shouldn't use the "same teams every year = most difficult" argument. Nearly every year, the vast majority of divisions have at least 2 medalists who were in the top 3 the previous year. There isn't as much turnover among the top teams as people assume.
Bottom line, all of the divisions are tough to win and it is tough for anyone new to break into the top tier of teams. It can be done, certainly, but it is not easy.