There’s a large ecosystem of iOS music making apps and the iPad is actually an excellent interface for some applications, e.g. a kaoss pad.

For instance have a look at what Korg and Moog have available in the app store. Or check out drambo.

I think the "no windows" and sandboxed apps/files paradigms are very limiting for pro work.

And you really need lots of customisable keyboard shortcuts for most pro work, most iOS apps are extremely lacking in this department.