This skill's most notable effects are the boosts for Sloth power and damage, since it applies to all allies for the turn. Assuming Yi Sang manages to get Sunshower out first by being the fastest Sinner, any allies using Sloth skills for the turn get a nice power and damage boost. The additional Protection included is icing on the cake, protecting allies from attacks if they need to tank a hit or if they lose a clash. It may not add to the damage allies can inflict, but it's universally good in helping them tank a bit more.
The best thing? All the effects mentioned above applies to this turn and the next, which means that if you really want to, you can go at it to use Sunshower for this turn and the next to stack the buffs given by the EGO. Sloth skills get boosted even more, and the added Protection will serve wonders for the team's survivability.
It has an additional effect of healing Yi Sang for 15 SP if the attack flips heads, but as the EGO consumes even more SP on use, it's best to not rely on its effects too much. Still, it can be very useful in helping Yi Sang gain back some of his SP so he may be able to use this EGO in quick succession if need be.
This skill's effects are noticeably different from the Awakening's version, as while the Awakening focuses on buffing allies for this turn and the next, the Corrosion's skill focuses on buffing Yi Sang himself with Sloth Power Up and Sloth DMG Up for this turn and the next, meaning that the Sunshower EGO skill that he's going to use will buff himself too. Not only that, the effect will last for this turn and the next 2 turns, making it 3 whole turns where Yi Sang gets the Sloth buffs. Considering the fact that the buffs can stack, Yi Sang can output ridiculous amounts of damage by just spamming the Corrosion version of the EGO multiple turns in a row. There is, of course, the danger that Yi Sang may dip to -45 SP and corrode himself, potentially causing him to use those super strong Sloth buffs against you.
There is also fact that this skill also inflicts Sloth and Pierce Fragility to all enemies hit by this skills which lasts for this turn and the next. This makes them even more vulnerable to Sunshower and allies that use Pierce attacks such as R Corp. Heathcliff and Cinq Don, or allies with Sloth attacks like Seven Outis or K Corp. Hong Lu, making it function as a neat support skill that debuffs enemies. Considering that Sunshower is an AoE skill that targets 7 slots at Threadspin IV (5 at Threadspin III), it's more than likely that every enemy in the field will be getting hit by this attack.
In comparison to how detailed the effects of his attacks are, his passive has a much simpler effect of healing every ally's SP by 3 per turn. It's not game breaking or anything, but having a constant source of SP regeneration from somewhere, even if it isn't much, is always welcome, especially if you're trying to use EGOs as much as possible while trying to mitigate the risk of corrosion, such as in floor 5 of Mirror Dungeon 2 Hard.
At a glance, Sunshower seems quite expensive to even use. 4 Sloth, 2 Envy, 2 Gloom and 2 Pride for a total of 10 EGO resources for one Sunshower usage. Overclocking this EGO means you're using 1.5x the amount of resources to get the Corrosion variant out. But as it turns out, being able to stack buffs and debuffs on both you and your allies means that you can ramp up on a lot of damage real quick, especially if you have a lot of Sloth skills for Yi Sang or your allies (and Pierce, if using Corrosion). The fact that it's AoE means that whether it's against a single enemy or a whole party of enemies, Yi Sang is sure to deliver significant amounts of damage while giving out buffs to the party and/or debuffs to the enemy, whether he uses Awakening or Corrosion, especially since the buffs and debuffs he dishes out directly buff his EGO, being both Sloth and Pierce.
As you might already see though, Awakening leans more on support, as it buffs Yi Sang's allies damage capabilities for Sloth skills while giving them Protection. Corrosion, on the other hand, is more selfish. It makes enemies more fragile to certain attacks, but the Sloth buffs are all going to Yi Sang, making his EGO use even stronger.
Its biggest strength is also its biggest weakness. The buffs and debuffs the EGO can give out last for multiple turns, but if you want to make the most of it, you'll need to use Sunshower multiple times in a row, which can eat up both EGO resources and Yi Sang's SP. And as mentioned earlier, a corroded Yi Sang buffed by Sunshower targeting your allies is a recipe for disaster and is more than likely going to heavily damage, if not outright one-shot almost anyone.
All in all, an amazing EGO and more than worth its weight in gold, despite its expensive cost to use. Just make sure to keep an eye out on Yi Sang's SP and your remaining EGO resources while using it. Or you can choose to ditch safety precautions and go all out if you think you can wail and defeat the floor 5 boss of Mirror Dungeon 2 Hard before Yi Sang has a chance to turn on you. For best results, use this EGO on his Spicebush Identity. He can fund almost all the sins required to activate the EGO, has 2 Pierce and 1 Sloth skill, and even has a passive that boosts his damage if he's attacking multiple targets, which Sunshower atuomatically does. it's a perfect fit for him.