Recently tank-assassins have been the meta
I'm not sure I'd say tanky assassins are the meta.
Certain assassins just find themselves in situations where building bruiser offers a greater risk/reward dynamic.
Irelia was originally released as an assassin BTW, so "tanky assassins" isn't exactly a new thing.
It would appear that tank items are also a lot more potent than damage items, but I haven't put much thought into that.
Tank items being more cost-effective than damage really depends on the situation, but generally speaking: HP is a lot cheaper than AD/AP, but AD/AP have much better multipliers for a strong end-game, and AD/AP allow you to farm much more effeciently so you have gold to invest in stats.
My personal thought pre-League was that tanks should deal very little damage but be able to take an ass-whooping. In league, this simply isn't the case.
This is really dependant on the game though.
In a game like FInal Fantasy, Earthbound, or Pokemon it makes sense that a tank is going to hit for peanuts where a glass-cannon is going to do tons of damage.
However: LoL isn't a turn-based game. Range and mobility are 2 very real, extreemly powerful means champions have to survive long enough to do their job.
The community generally agrees that mobility > defense.
Looking at Marksman being 100% required and champions like Yasou/Fiora being incapable of fullfilling the same niche, it's also worth considering range is an extreemly powerful stat to have.
Which means in LoL: champions can survive long enough to do their job through tankyness, mobility, and/or range.
As such: champions with no/extreemly limited range and mobility like Garen doing tons of damage isn't broken.
I feel like there should be a much more solid line between tanks, juggernauts, and carries.
LoL has over 100 champions, and some of them are like Kayle, Nidalee, and Urgot.
You're not going to have solid lines between archetypes, there's a lot of blending and cross-over power with cousin archetypes.
Currently, Juggernauts do as much or more damage than carries, and are just as (if not more) tanky than dedicated tanks.
Juggernauts are tanky DPS, tanks are tanky CC.
The difference between the 2 isn't their level of tankyness, it's what they're trying to accomplish outside of being tanky.
I don't know how to fix this other than separate damage and tank items entirely, but this is just my thought - perhaps guinsoo's and devourer aren't the issues, but tank-damage is?
In regards to tanky assassins: main issue is that assassins are inherently risky to play, and with Riot's persuit of counterplay: many have such huge telegraphs and work-around they're risk/reward dynamic heavily favors just playing them as a mediocre fighter rather than a terrible assassin.
In regards to Juggernauts and fighters doing damage: Those are tanky DPS archetypes, you're Barbarian from Diablo or Dungeons and Dragons. They're Damage and kill-heavy rolls just like assassins, marksman, and mages: they just rely on defense to survive long enough to do their job rather than range or speed.