I know Soul Exchange was too slow in the normal, non-Yubel version of Phoenix deck, but has anyone tried running 2-3 copies in Yubel deck?
It adds an additional source of monster removal and lets you summon Phoenix even more easily.
Perhaps you could even use instead of Fire King Island, so you don't have to worry about your Terror Incarnate getting destroyed when you wanna bring your Phoenix out on the field.
But Fire King Island gives you access to Terror right away, that's the whole point. You really don't want to be summoning base Yubel with it's tribute demands.
You can get over having Terror destroyed by Fire King Island by playing Interdimensional Transporter. Or you can just let it be destroyed and get a free Nightmare. You can always just destroy a second Yubel to get back Terror.
I'm not talking about Soul Exchange summoning Yubel, I'm talking about Soul Exchange summoning Phoenix, so you don't have to deal with the annoying Fire King Island effect.
And, there is a card releasing a while back that can search directly for Yaksha when destroyed.
Barong searches for Yaksha and Fire King Island, that's why it gets played.
Plus it's generally advantageous for Phoenix to be destroyed by Fire King Island so you can trigger it's effect. You've got to use stuff like forbidden dress to protect Yubel from the destruction effect.