Juggernauts are balanced around their low mobility, low CC and/or low effective range to compensate for their high damage and durability.
Generally, they have to be within melee range of a target for fight them effectively, but they lack the tools to effectively get within melee-range of their target. For example, Volibear has a single mobility tool (the MS buff on his Q) which requires he run up to his target (often requiring he go through the enemy team to get there).
As such, he is rather vulnerable to kiting through slows and CC; a peel support (i.e. Nami, Janna, Lulu, Zilean) can often be enough to negate him during teamfights even if their ADC or other teammates can't kite him by themselves. This is ignoring mobile ADCs (i.e. Lucian, Kalista) and CC-oriented ADCs (i.e. Ashe, Varus) and mid-laners (i.e. Azir, Vel'koz, Lux, Leblanc) using their own tools to kite him and the fact that he has to get past the enemy tanks/fighters first to even have a chance at reaching the back-line. Rylai's Crystal Scepter and Frozen Mallet are also very good tools for kiting him as a squishy target (while givign
Volibear in particular has issues dealing with durable targets. While he has high base damages on his W and R (provided you can stick to a target), he can't effectively itemize for penetration items (i.e. Black Cleaver) like other Juggernauts (i.e. Darius, Garen, Illaoi) can. This reduces his damage against tanks and fighters immensely, so he needs to reach the back-line to be effective and he has few tools to do so.
A definition for Juggernauts (and other classes) is available here:
Juggernauts are melee titans who relentlessly march down the opposition and devastate those foolish enough to get within their grasp. They are the only subclass who excel at both dealing and taking significant amounts of damage, but in turn they have a tough time closing in on targets due to their low range and extremely limited mobility.