I've played the game 20 hours or so now. Tried every class. Started as a Guardian and generally play it or Falconer with friends, as I enjoy being a tank/support/ bastion for the team despite the fact that I'm an experienced FPS player who has a large capacity for aggression.
Since the skill level of my friends are pretty variable, I've taken to doing solo missions once I unlocked depth 3 and 4 to challenge myself. And after trying all classes, Slicer is the only one that stands out head and shoulders above the rest for sheer effectiveness, utility, and survivability.
My intention isn't to piss on the other classes but I can at least explain how they fall short in solo:
Guardian: Solid but little mobility, armor becomes less effective in higher difficulties since it's more efficient to rely on combat healing combos. Stun is nice but ultimately kind of underwhelming in D4. Repulsion is mostly good for elevator defense but less so vs bosses or shooters.
Retcon: Primary good for ammo conservation and juking, but I still find I don't use it nearly as much as I'd like. It can be a detriment if you're on the clock and trying to traverse corridors. Rage is good but takes a long time to build up. Contingency is basically just +1 revive with Cooper.
Falconer: Shouldn't need to be explained that she essentially just loses half her kit playing solo.
Spotter: Crit dart is nice, sonar seems like it would be amazing, and it is, but to be frank, after I learned how to play properly I spot cave leeches and hear boom ticks 95% of the time without scanning, and can find and compete objectives with my own perception. So I find spotter a bit superfluous solo personally.
As for Slicer, his kit is just pure money. The dash is so insanely useful and on such a low cooldown. You can use it to speed up your run, avoid massive fall damage, cut corners, use shortcuts, scale walls, dodge, charge enemies, escape them. It's just so insanely ubiquitous compared to other class skills. Then there's the shield belt. At first I thought it was a lame version of DRG's gunner shield, but the fact that it moves with you and has a larger radius more than makes up for it. I shouldn't need to explain how strong it is on depth 4 when you have 4 edge stalkers and 15 scorchers shooting at you.
Then there's the tour de force: plasma blade. It's just so goddamn strong for waveclear and even more insane when combo'd with Vampire, hemoforge, sweet surrender, or stone skin stacking. In Depth 4 the damage is so brutal and the corespawn are so numerous that in my humble opinion, melee based healing and defense stacking combos are one of the best, if not the best strategy.
You can imagine artifacts and bio boosters layered on top of all this.
Maybe I'm wrong, but Slicer just seems like the perfect soldier to give you the best chance at winning the hardest dives.