I think a steep learning curve can be ok for any design if it suits the circumstances. Lots of expert systems are hard to learn but work really well once you know them.
It’s all about the goals…
Which is to say that novel interface controls in software instruments can be awesome, but only if people have a reason to invest in learning them. For software instruments where the creator wants to give people an entry point using familiar elements is a good way to do that.
I think it could also be argued that software instruments, as opposed to hardware ones, require a different kind of learning as they can be less “explorable” … a piano or guitar make noise and have physical cues that help you figure out how to play them even if you’ve never seen one before. A lot of software doesn’t do anything unless you use it “correctly” and the feedback mechanisms that promote learning by exploration/play have to be intentionally designed into it or it won’t exist.