Backloaded, most tanks have become backloaded from mobility creep denying their effectiveness; and like bruisers, still don't get off more than a rotation of spells unless they're outright ignored. Tanks aren't as dangerous as bruisers, but more ignorant players underestimate their potential; defense still has diminishing returns, while physical damage doesn't thanks to multiplicative scaling. Bruisers still crush tanks when it comes to split pushing, and can successfully duel other champions more often.
The removal of runes also stripped the vast majority of defensive stats people relied on to survive trades, more-so regarding magic resistance, which the majority of tanks deal when it comes to damage; and the mastery change has pathetic compensation for it.
What tanks are frequently complained about recently? Chogath, Maokai, and Malphite. But do you know why?
1 Chogath's area silence counters the spam-able champions like lux/riven/ryze/yasuo, while having reliable damage against someone who doesn't respect his melee range. Assassins rely on abilities to be effective, but are also needed for the marksmen menace; however both classes can be crippled when he's close enough. Know what's easy to max out in a game these days with only a few items, cooldown reduction, to the point some players bank on the reduced wait window.
2 Maokai has some of the last point and click crowd control in the game, a necessary evil since riot refuses to rework some degenerate champions like azir/kalista/leesin/vayne, who can practically juke skillshots at will. Despite loosing his damage reduction, they increased his sustain noticeably, allowing him to function as a magic bruiser in some situations similar to gragas. Healing has always been a catch twenty-two because of grievous wounds; either it's too good without it, or not enough with it, rarely has there been a middle ground regardless of the class sustaining.
3 Malphite has sudden aoe crowd control with good reach, and unlike the previous mentions, ignores mercury treads resistance to consistently deny enemy action for the duration. He has armor ratios on his abilities to benefit more from building defensively; what's popular lately, lethality, which is countered by armor. Newer players frequently complain about why do tanks/bruisers get damage for being durable; when the flipside has been just as troublesome to balance for years, issue items like death's dance, phantom dancer, and lifesteal items in general preventing squishy charecters from a death short of telefragging or stunlock.
Where's the other tanks such as mundo, poppy, or volibear? Outmoded, obsolete, ineffective; in comparison to others who can crowd control better than them, or survive more than they can, or setup allies reliably, or some combination there of.
Remember the tanky assassin wave, it's because some problematic assassins had high base and ratio damage so they didn't need much to kill with, while offering some crowd control allowing them to take the place of a tank without sacrificing anything. Why play an immobile tank that can't kill, when i could play someone who is both quick and dangerous, and just build them durable enough to deny being shut down? Riot had forgotten why some archaic things had been balanced on their side as it were; such as old poppy being terrible early game because she could do too much late game, then they made ekko who repeated those problems all over again. Partially from tanks being directly nerfed at the time, partially from some overloaded items being left unchecked, but mostly from overlooked design issues in assassin kits.
Lets return to damage vs durability. Marksmen have damage, attack speed, and critical hits to combine their damage output; tanks on the other hand just have armor and health. In the current system the former will always outdo the later, unless there's some kind of extra edge. Given riot's anxiousness to fast forward the game as much as possible, this isn't going to change either. Neither is it looking like they'll return to clear weaknesses, since they continue to enforce/make/remake champions in ways to overcome their flaws, save for gimmicky minigames. A great example would be the change to guardian's angel, making it even more beneficial to the ranged rapid attack carries, than it was before as a ditch effort niche item.
How about an example for more discussion?
https://steamuserimages-a.akamaihd.net/ugc/866242507560938373/300353EBCAFBAAD640EEDC6704852D7ADC7E96FB/
