Slim is broken. He acts like a one-man army. This is primarily due to his team permanent cloaking with the Spore Cloud and the fact that he can also heal faster than anything but a Stage 3 Monster. He also has two healing abilities, and all four of his abilities are completely spammable.
There are several detailed examples, including video, of the Spore Clouds in this thread:
The main conclusion I came to in that thread is that you cannot tell who is who in the spore cloud and it can be impossible to find a downed Hunter. This means Slim has a permanent defense, which is completely imbalanced compared to the rest of the Hunters, and especially the Monsters with no defensive capabilities (apart from Behemoth).
But worse than this is the healing. Slim can outheal anything but Stage 3 abilities with direct hits. Why is this? Because his shotgun allows for 1-4 heals per cartridge. That is an insane healing rate, especially considering he has a second healing ability.
This is a very poor mechanic. Generally there are tradeoffs in FPS games: high rate of fire, low damage, low accuracy. Low rate of fire, low control, high damage, high accuracy. But what does Slim have? High rate of fire, high control, high accuracy, and high damage (when considering his healing plus what his aggression allows the team to do).
This simply does not make sense. There should be a tradeoff. High healing should go along with low damage down range. This is the tradeoff with Val vs. Caira, or Laz vs. Caira (high damage or high healing), you have to choose. Same with Cabot vs. Hank, you either get the damage or the shields, not both.
Slim simply gives you both options at once, along with giving invisibility of a Support, and aggressive pressure like Assault. He acts as four classes in one, and that is the epitome of poor balancing.
As evidence you can quickly go into a single-player match to see the potential healing speed that he has, or find a player with above average accuracy. The Monsters are quite easy to hit, so this is a game mechanic which needs a fix and rethink for other aspects of the game.