Dig deep enough? There's been years worth of complaints pointing out a handful of issues. Some of the problems are direct design flaws, others incidental oversights, and occasionally ignorant co-operation between the different branches; but usually a combination.
So let me elaborate for you.
Marksmen get too much damage too fast for any other class to meaningfully compete unless some cheesy strategy is used; critical hits specifically turning them from dps threats to physical bursters, often hitting quadruple digits on a single auto-attack once they get fed.
Reduce their damage output in some fashion, whether slowing their power curve or curbing their overall power. Rework critical hits, or lower damage or attack speed on their items. Or significantly increase the costs of their items, since they already have some of the easiest time farming.
Lifesteal is disgustingly prevalent, applying both on critical hits and repeatedly from boosted attack speed, generally prevents them from being killed by other dps dangers; and practically requiring stun-locking or even more burst to kill them.
Lower lifesteal or remove it's boosted interaction with critical hits, or make it an overtime effect similar to the bleed on death's dance.
Safety nets! I remember back in season one through five when they had to be protected by their teammates in a fight. Thanks to tacked on effects to their already powerful primary and niche items, they get increased windows of power via extra damage reduction and delay, ontop of guardian's angel being reworked to benefit them. A few niche items like essance reaver and runaan's hurricane should've stayed on the sidelines, as they can give way too much influence over the game depending on the marksmen. That's not counting the extra movement speed on a handful of items, or several activatable items to give them even more of an edge against any competition.
Undo the excessive buffs from the items in pre-season six (as I doubt they've kept previous states of the reworked champions), this alone should be enough to pull them back into line with the other classes. Either remove the bonus movement speed, the free damage reduction/delay, or the extra effects on their counter items. Things like maw of malmortius and phantom dancer did their job well enough before getting buffed; while other spiteful designs like dominik's regards and guardian angel, are just double standards, allowing one class to screw over another for doing it's job.
If an item is good, marksmen will eventually abuse it, then riot guts the item instead of their interaction with it. It noticeably started in season two with ezreal, and it's become a more obnoxious as time has gone on. Ask juggernauts about sterak's gage, or tanks about dead man's plate, or assassins about edge of night.
Drop the favoritism; make items melee only, or change them to rely on bonus stats so it benefits intended classes significantly more.
