Conclave, combined with shattered retreat and coalitions, has ruined the game for me. It has become extremely easy to increase your power through legislation, and I hardly ever see an HRE which is not an absolute monarchy within 80 years now.
Conclave allows you to exclude all the most powerful vassals from joining factions for most of the game. Shattered retreat causes fewer casualties. The result is that factions hardly ever become threatening. The only time you might feel exposed, is during regencies or when you choose to allow factions.
So instead of making the game harder by making factions more powerful and challenging, the game is now easier on the interior level. The limiting factor is now just the anachronistic and tedious coalition system.