Of course it's risky, it's a glasscanon build, that's why it's not the principal one. There's some counter, like Braum, Janna, or even Alistar (that one is an iron wall) that can and will (or at least try) to protect their ADC. But the thing is that after reaching your Youmuus, you can one-shot with your ult, and you can manage to escape because the vision radius of the enemy team is greatly reduced, so the enemy support will have difficulties to protect the carry. And most of the time, you can dodge 1 (or 2 with EotN) spells and then kill the carry

Yeah, mistakes were made... The trinity force isn't a really important item on this build, even it adds a massive burst and a nice AS boost (you got those already), you can replace it by a Maw or by a DMPlate... Now about diving in the ennemy backline : You will never ult their fed caitlyn if she's next to this mega-tank tree-looking maokai... You want to ult her when she's going back, and right after the beginning of the teamfight, by staying on the back, waiting for their tank to engage the fight. And finally, if there's an heavy peeler who always stays next to his carry, your double banshee will block his spells. But honestly, if you dont miss your combo, there's no way you can fail the misssion.