I seen some high elo players and streamers buy crit on assassins such as Zed. It proved to be quite effective, as specially for Zed when he was able to crit for 1.3k dmg with bonus passive dmg.
What do you guys think?
They don't really auto attack that much so it isn't worth it. Getting a shiv is fine sometimes, but that's really it. If you're fed or really late into the game, then sometimes an ie can work, but not often
People do/did situationally on zed/talon/rengar. I don't really keep up on the trends though.
not really on any other assassins unless I'm forgetting any
If they're snowballing it can be ok, particularly if they're one of those champs who has guaranteed crits built into their kit somewhere.
Other than that, the issue is many assassins are doing a sizable chunk of their damage with their abilities which (usually) can't crit, so they get more effective damage against armourless targets by building pen (in the form of lethality). Their kits are often pretty single target focused as well and designed to get in and out quickly rather than sustained damage, so the expectation is they will be free to choose an enemy carry instead of needing to deal with a tank, so lethality is the better choice there.
If they're super snowballed, crit can allow them to also be a threat to tanks, even if they might be doing slightly less reliable damage to squishies. If they're snowballed though, they'll still be able to pop them.
Build paths are probably also a consideration. Stacking a bunch of longswords->dirks/warhammers is much easier than saving up 1300 for a bf sword. Flat ad/lethality also is immediately effective, while crit is quite unreliable damage until a few full items, and assassins are all about trying to snowball early.
Because as someone else said, mainly AD casters, they rarely auto attack. Also, crit Zed was shoved out of the way a LONG time ago. It's not as good as it used to be.
Idk I think throwing a shiv into a khazix or zed build could be interesting.
They used to be able to until the Assassin rework came out and removed most of the Auto Attack resets on AD assassins like 
and
as well as the crit component to 
which was crucial for crit builds. You needed auto attack resets to compile the crit so you could at least have 1 and potentially all of your auto attack be a critical strike.
Now any AD is locked down into Lethality builds as there is no reason to build anything past 
for crit builds. The attack speed is underwhelming to build but refreshing to have when you're clunky ass auto attacks become buttery smooth!
it's unreliable and also usually you want AD scaling and lethality for your abilities
Potentially too unreliable? Assassins don’t normally AA that much because they’re mainly caster champions. Unless they have guaranteed ways to proc crits, it probably isn’t worth it.
Because most assassins are basically AD-casters. Assassins who use their AA 
do build crit
