Stattik Shiv is the main problem with marksmen
Now, I'm not saying there's not any problematic champs that don't use Stattik, but Stattik is just too good of an item.
Stattik, when it crits, does as much magic damage as Duskblade provides melee champions with physical damage, but without the sight restriction, so you don't risk losing it if the enemy is warding.
And here's where it gets problematic. Shiv lightning does as much damage as a same level Duskblade when it crits (Unless you're Jhin or Yasuo), without an Infinity Edge, to up to five people. With IE, it's a max of 480 base magic damage on a Stattik crit proc, in comparison to the Duskblade's max of 320 base melee physical, but ranged champions get less damage than melees do out of Duskblade's proc.
But with IE, Stattik will always do more damage than a Duskblade (at the same champion level) proc when it crits, and can do it against up to 5 people at the same time. That's just plain massive. It's effectively having a multitarget Duskblade proc, in addition to all the crit damage you already do. And it also has 65% bonus damage vs minions, to help Marksmen waveclear more efficiently.
And because it stacks with rapidfire cannon, you can land it from a pretty long distance as most marksmen. I personally think rapidfire cannon should be buffed, same amount of damage as current stattik and can crit, but made unable to stack with Stattik, while stattik should still be able to crit and have the bonus damage vs minions, but has less overall damage on procs than current Stattik.
And with that change, which would pretty significantly tone down most of the more problematic marksmen to a more balanced state, I feel would come a good opportunity to buff the struggling Marksmen, Lucian for example, so that they can be viable too while also toning down the problem of some particular Marksmen having too much burst damage without making any tradeoffs.
- TLDR: I think Rapidfire should be buffed, and think Stattik should be toned down a little bit, but also think that Rapidfire and Stattik shouldn't stack. And struggling Marksmen should get some buffs as well as compensation.