I can agree to this: out of all the game's DPS classes, if there is one group meant to specialize in AoE damage, it's juggernauts. Marksmen already have excellent target access, and so do not need to be able to purchase the option of heavily damaging multiple enemies at once, at least not without making severe tradeoffs. This is not to say that no marksman at all should be able to deal good AoE damage, and I think there are quite a few who definitely have that as an asset (e.g. Miss Fortune, Sivir, Twitch, etc.), but that additional strength needs to come at an equally steep cost, which isn't always the case: MF has less single-target damage than most marksmen, and Sivir tends to have less overall raw damage and range, but Twitch is an excellent duelist with a ton of burst, in addition to great DPS and AoE, which makes it difficult to identify a true weakness on him.
The issue I have with Runaan's Hurricane is that, while it's definitely more functional than its previous iteration, it's an effect that provides a lot of power without real gameplay, effectively allowing the marksman to deal a ton of collateral damage while focusing a single target. In the case of on-hit marksmen like Kalista, Kog'Maw, Twitch, etc. it also removes a portion of their counterplay by making them a threat against multiple enemies at once, instead of allowing a single champion to approach and soak up focus while others attack them. In addition, it gives a ton of additional waveclear to a class that already dominates in pushing power, and when coupled with crit and life steal it adds yet another multiplicative scaling effect that makes their damage and sustain go even more over the top: Hurricane alone, for example, increases the power of a marksman's life steal by 80%, which I think is largely responsible for their self-healing being as excessive as it is right now.
I also have become less of a fan of the Kircheis Shard line over time, not just because of Statikk Shiv's AoE (like Runaan's, it gives good waveclear to a class with top-tier waveclear already, and lets them deal a ton of AoE damage with little risk or effort involved), but also because of the item's innate upfront burst: while I think it's okay to charge up for an empowered attack over time in a fight, it's that initial amount of extra damage, coupled with crit, that causes marksmen to go way overboard and deal those ridiculous 1k+ damage autoattacks right off the bat: with Infinity Edge, Statikk Shiv's proc deals 400 bonus damage to 5 targets at once, which in itself resembles a lot of mage nukes (Orianna needs 267 AP to deal just that much damage with her W). It basically gives marksmen an extra nuke ability that members of other classes would normally have to pay a mana cost and aim to apply, without interrupting their attack pattern.
Considering how these items are all crit + AS items, I think this also raises the question of how we want to implement crit in the game: this is probably a topic for another discussion, but a major issue with marksmen right now, imo, is that they get to kill squishy targets too fast, in addition to being able to kill multiple targets at the same time without much of a risk or tradeoff. While the Marksman Update added some much-needed diversity to the class's itemization, I think it also failed to address many core issues with the class, namely their reliance on an RNG stat and the lack of strong differentiation between members of the class (there are still no official marksman subclasses, for example). Drawing a harsher line between marksmen capable of dueling, and marksmen capable of AoE/teamfight damage could be one step towards allowing those differences to express themselves better.