So here is the thing I've noticed so far, All the Swarm champs (Malz/Zyra/Annie/Azir/Zed etc) are way too strong early for the cost; Malz/Zyra and to a lesser extent Azir are all very very very strong early and it's pretty borderline being "The best" option if those choices come up you almost have to try and sit on them to get them to plug in.
Malz/Zyra are way strong early and for whatever reason the Zyra's that are (i believe) 1 cost actually do a lot more dmg than the Malz voidlings that are 2 cost..Tibbers is also really strong but at least Annie is a 4 cost and it's kinda tough gettin him out sometimes so it's like ok that's not too bad. Azir seems the most ok or maybe undertuned compared to the others and he is kinda rare to find so i don't have a huge prob with him.
You look at the Woodland bonus and when you get something really silly like ivern maokai and either lb or neeko who are all really low costs, easy to find and they are perfect for swarming and giving utility it's like these are the easiest champs to get and get consistent results.
you won't win every game late or anything but it makes getting top 3 pretty simple when you get a swarm with 1 or both options.
Honestly i like that they aren't on the level as the dog shit Elise or anything like that, maybe the duration needs to be lower? since the bonus is extremely strong when you start putting tear items on some of these champs it means they can get a lot of minions out early and that's fine for the game but because there are really limited AOE options in this Set in terms of champs and items it's maybe a bit much.
Berserkers are cool but they are actually tougher to get going since many of them are higher cost the easiest counter to swarm would be that imo but scarcity makes that kind of a tough option.
I can't really see any other hard counter than that; there are only a handful of champs now that have base AOE/Cleave dmg built into their Kit so that means Swarm is going to be really strong until either the Duration is brought down or other direct counters are easier to obtain, otherwise it's all just about swarm mid and maybe get some sort of inferno or something later.
More engaging then the first set obviously but still it's like ehhh this isn't too great
I will restate though, late game they are for sure pretty beatable if you didn't get some great items and have multiple lv's up on your champs. Taking a decent lead into the late game consistently though is what i've really noticed, almost always you can build a really dramatic lead and by the time it gets to the top 5 you are still sitting pretty healthy while most others are pretty low. Even with sub-optimal items you can finish top 5 against average players pretty consistently from what i've experienced anyway.
Every time I used Swarm i got 2-2-3-3 and a 5 but whatever to that 5. I gotta say it may possibly need to be tweaked.