Helldivers 2

I'm not sure if the Quasar will take away my Expendable Anti Tank slot, so I might not be the best one to answer your question. But I can see some advantages over the autocannon.

• It doesn't take a backpack slot
• It can one shot a lot of stuff
• There's no reload time, just a cool down time. So you can switch weapons and continue to fight/run
• Unlimited ammo is pretty nice

Edit: I realize I didn't quite answer your question since you asked about situations. I also haven't used it enough to get a real good feel for where it shines.

I am 100% a Quasar enjoyer, personally I like the cadence of it. You pull it out behind cover, let it spin up, shoulder-peek and delete something, then put it away for 10-15 seconds and it's ready to go all over again. No backpack slot, no ammo concerns, no reload animation, no waiting for a call-in to deal with a heavy. The Quasar projectile has no projectile drop either, making it hands-down the best weapon in the game to snipe bot dropships or distant objectives. I'd be reluctant to "waste" an EAT or RR shot on a Devastator, but the Quasar's infinite ammo means I'll happily zap one on that 15-second cadence if there's nothing bigger to shoot at.

One piece of advice for using the Quasar: don't use Aim-Down-Sights. I use ADS with most guns in this game, but not the Quasar. If you've only used it first-person then you might not realize that the third-person reticle for the Quasar gives way more information. The reticle spins and slowly turns from a square into a circle when it's about to fire, making it easier to time your shot when shoulder-peeking. The reticle turns red while it's on cooldown, white when it's good to go again. The ADS view gives you none of this information visually.

My group runs 8s if we feel like pushing ourselves, 7s if we want a less frantic pace. At those difficulties there are a lot of heavies to deal with, and the Quasar feels like the most flexible tool for dealing with them. The RR can shoot faster with a team reload, but that requires coordination that can be hard to manage under pressure. The EAT can give you two quick shots per call-in, but then you're left with a minute-long dry spell before the destroyer can give you any more rocket goodness.

The Autocannon will tend to struggle against Tanks and Hulks. That said, if you enjoy the AC then go with what you like! One of the regulars in my group usually brings an AC, and I'm happy to have them there because it can deal with other targets for which the Quasar isn't suited. Against bots the AC shreds walkers, against bugs it pops spewers, and against both its splash damage will clear chafe packs quickly. I wouldn't want a full team of ACs on a 7 or 8, but one or two are always welcome in the mix.

You can one-shot drop ships out of the sky, causing it to drop on top of the bots that were just delivered: I get 10-13 kill streaks with one shot. It smokes a hulk in one shot to the face, and 2 shots to the front of a tank turrent.

On bugs, you can blow a chargers head off in one shot, kill shrieker nests in 2 shots, and blow the head off a titan in 2.

It's an absolute monster weapon that borders on OP, which makes me think Joel is about to shaft us with way tougher enemies soon.

It's an infinite recoilless with long charge up time. That's it. If you are generally positioning away from fights, have trouble finding the right moment to reload the recoilless or have bad aiming, it's good/better than them. If you are generally frontlining, the charge up time will get you killed in situations where a recoilless, which is instantly ready to shoot, or the EAT that readies up in less than 1 sec (and you can do it while running) will outshine it.

It's not in the same category as the AC at all. But if you want to vary things up, as a fellow autocannon enjoyer, I would recommend the arc thrower. It kinda fills the same niche of anti-medium/anti-hordes. It's much better at anti horde duty than the AC but a little worse at dealing with medium armor, and has less range. Which it trades for better multi-target damages and zero reloads/ammo management. Also positioning is more important than with the AC to set up good arc chains/not kill your friends/avoid the arc getting blocked by terrain, dead bodies or bushes. Also it will still deal some damage against hard targets where the AC just bounces off. For example I find it easier to kill chargers with it than the AC because you just have to arc them in the face 5-6 times.

No backpack for one. A shield gen is great for survivability, a jump pack is godlike for getting into places where nothing can get you. Shuttle defenses become a breeze with quasar + jump pack, since you don't need to restock its ammo (you can't drop stuff on mountains and cliffs, it bounces off). Three people with quasars can reliably shut down a bot drop by shooting dropship engines, you can kinda do it in two, but sometimes there is like 5 or 7 drop ships and you need to grab some air, or someone misses. Also tanks get dropped and always survive the drop even if you frag the dropship, so someone needs to fire a shot or two at them. One hit on the charger, easily takes out bile titans, tanks and the dreadnought-looking enemies (with a possible one shot to the battery or the tiny head on the front). You can use it on annoying enemies of "lower" threat level too, since yaknow infinite ammo. The downside takes a tiny bit of getting used to, but after using it for a day or two, you are just as effective with it as with EAT's, but you don't only get two shots.

Plenty to like about this weapon when running AT.