Ussually don't play survival islands because they lack all the biomes but played one and am finding the mineshafts to be better. Why you ask? Because they are under the ocean so they are at a lower elevation. That means you can explore the mineshaft and look for diamonds etc. at the same time. A lot of the mineshaft lowest levels are at bedrock.
Everything should get generated on the maps now. I think they have fixed all the bugs. Only superflat worlds don't get that stuff.
By the way the seed I'm using is The Perfect Form
It has about 20 islands and half of them are mushroom biomes. Extreme hill islands for many of them too.