Here's what usually happens. First, the division just before the next league (<insert league> I) is filled with shitters, no exceptions. Most are there because they've hit their ceiling. Second, the division at the bottom of any league is just as bad as the division at the top of the league below them (Gold V = Silver I, Plat V = Gold 1, etc. etc.).
But most importantly, you don't have to go through the shithead process of winning a bo5 instead of a bo3 to advance, so it's the exact same difficulty without all of the horseshit of a bo5.