If we're going to judge kits by how PUGs play them, then the Juggernaut is the worst kit in the game, the damn things only ever spam siege pulse, even when they have a bunch of enemies on them, and I get my 15 revives medal super early in the game.
No, but seriously, though, the Slayer is a very versatile kit. Yes, you get a ton of idiots dying by spamming PD without ever charging to get those shields back, or idiots standing out in the open to BS when they should have charged to get shields back. But that isn't a good representation of what the kit is capable of. Are Kroguards squishy because people fire off Carnage instead of charging to get their shields back? Is the cabal gabage because people try to poison strike entire waves instead of a more tactically sound option? Are human engineers nothing but Spitfire/Javelin users who can send out a new combat drone once every 30 seconds?
If you dislike the acolyte (And I do personally feel it's horrendously overused - the slayer is one of the very few kits I actually field it on) The Arc Pistol is another substitute you can use; basically just good in general. The Talon also fits this role nicely due to it's Shield/Barrier damage, allowing you to spec PD for armor damage without any worries. Or, if you prefer SMGs, almost any SMG works well, since you can also slap on the Expert Package to reduce cooldowns a bit, even though I'd suggest the Geth Scanner just to Slash through walls.
If your concern is people spamming Biotic Slash the whole game... I actually don't see the difference from literally any other kit. Every kit only has 3 active powers, often technically less if it's an activated mode like Bloodlust, Devastator Mode, Biotic Focus, etc, and at most, two guns. Every game already has a lot of repetition. There's a time to slash and a time to charge. The two rarely, if ever, overlap, so yes, you'll definitely have times where you slash 3 times in a row, but is that any different than unloading 3 clips of your gun?
Really, though? Like most vanguards, it DOES take skill to play the slayer well, doubly so considering it uses it's barriers to attack. It's good that more people don't play it, because like with literally every other kit, they'd suck. But in the hands of... well... someone who has enough common sense to take cover instead of standing out in the open using Slash when surrounded by Marauders? It's pretty damn good, and my easiest Gold solos ever were with this kit.