Slim is currently hovering around a 60% win rate, though some of that is probably Sunny’s doing; she’s quite strong, and they tend to be taken together. Regardless, he’s far from trash, and unless I’m much mistaken the general community thoughts on him tend to be that he’s the only Medic other than Caira worth taking (unless you’re going for some niche hyper-aggressive comp with Val).
His healing is… actually quite good, if you know how to position. If you think charging it up is “ridiculous”, then you’re probably not using the Leech Gun well enough. Try getting closer. He can easily get two or three healing bursts off every four or five seconds. To compensate for this, this requires him to position very aggressively, which makes him vulnerable, so you’ll generally need to coordinate with your Support to defend you. It helps if they’re able to actively shield you; Hank and Slim work very well together.
Shady the Magical Healing Fairy doesn’t like getting close to the Monster. He runs away if the person you send him to takes damage, so he’s strictly for keeping the back line topped off or reviving downed teammates. Especially the latter. You should always try to send him towards anyone who’s been knocked down. The Monster often won’t spot him in time, and you’ve got a free revive off as a result.
The spore grenades are actually very disruptive, but this is admittedly something that the Monster can learn to mitigate with practice and depends on your allies being evasive. In general, you should open fights by covering a wide area in spores, then switching to the Leech Gun if someone needs the extra healing. Whenever you see the opportunity, though, put down a spore canister. Keep in mind that it’s best not to aim directly at the Monster, but rather at the ground between the Monster and whoever you think his next target is going to be (unless he already has a target pinned). This will keep him in the cloud as long as possible. Maximum bang for your buck.
Really, Slim’s healing is very strong. It just requires you to play him aggressively, which is dangerous and requires your teammates to pay attention and be ready to assist you. Remember that Slim gets reduced healing from his own bursts, so he’s not as tanky as Caira. But so long as you pay attention and are ready to dodge when the Monster turns towards you, you’ll be quite easily able to fulfill your role in the team.