LOL, you just named 3 of his worse match ups. If you're having a hard time with those 3 characters using Sagat, you'll still have a hard time with them using Evil Ryu.
Seth: I think it's actually even, annoying but even. Evil Ryu actually has a corner and mid screen unblockable against Seth. Land one of these and it's basically a wrap.
Ibuki: Really bad match up. When knockdown'd she gets unblockables and teleport gets punished easily. From full screen or mid, she can react to fireballs with neckbreaker or slide. Hard to anti air if she does a early kunai from forward jump. You're force to try to get in her face, but that's not easy in itself.
Viper: Also pretty bad, she does so much damage and stun and he has low stun. The biggest problem for Evil Ryu is that he can't low profile her air burn kicks like other Shoto's can. Not even sure how to approach this match up, I just pray that I'm on the lucky end of random shit.
Akuma: Again, pretty bad. He's just better than Evil Ryu in almost every way. Faster walk speed and normals help him dominate footsies. And Evil Ryu can't do anything against well spaced neutral and jump back air fireballs. I think this is actually his most winnable of the 3, but you'll need a lot of patience.
Yun is my back up character and I would use him to counter Viper and Ibuki. Against Akuma I tend to still stick with Evil Ryu just because it's a shoto match up. So it's something I feel comfortable fighting, even when it's bad for me. In Ultra I think the Akuma match up will be close to even because DWU really hurts him. But I still think Viper and Ibuki will be bad for Evil Ryu.