You’re obviously talking about GLES2 devices there. I have an iPad Air 2 with GLES3 which can do at least 2^24 vertices (actually most GLES2 devices have the extension GL_OES_element_index_uint which enables 32 bit indices.
And up to 75 bones ? This is probably again a GLES2 limitation. In GLES3 you can have 12 x 65K uniform buffers which amounts to at least 12 000 bone Matrices. You want to say that Unreal doesn’t check for the existence of GLES3 in order to use its capabilities ?
EDIT : After doing a quick search it seems they just create a ES2 context on iOS, there is no reference to kEAGLRenderingAPIOpenGLES3 