By "bruisers" I feel you mean divers, as juggernauts are bruisers (or fighters, as they're usually called) and don't have the mobility profile of assassins. Beyond that, I agree to some extent, and I think divers as a subclass tend to be overloaded: whereas most other subclasses tend to specialize in a handful of strengths, divers generally have a spread of mobility, crowd control, DPS, burst and durability, without really excelling at any of these. When divers are especially strong, as was the case in Seasons 2 and 3, they tend to overshadow tanks and even some assassins, because they get to supply as much assassination and tanking potential as is needed from either class. When they're less strong, there isn't really a particular reason to pick them, as every other class collectively ends up doing what they do better.
Because of this, I think divers need to be pared down to far fewer strengths, and should also have much clearer weaknesses as a result. Having less durability than juggernauts or less burst than most assassins isn't a real weakness, it's just a bunch of poorly-defined strengths. Seeing how burst tends to be the domain of assassins, and crowd control the domain of tanks, I think it would be healthier overall if divers, as a general rule, specialized in DPS, durability and burst mobility: as a result, they should be able to reach the back line, damage priority targets and survive for long enough to be able to win fights there, but they shouldn't be able to assassinate targets in the manner Jarvan or Wukong can do right now with Lethality builds, nor should they be able to CC a target for as long as Xin Zhao or Jarvan in their current state.