While software might be more complex (aka, has more features packed), I think I do not agree it is more difficult or harder to test, it should be easier actually. If you take Tonverk as an example:
- The codebase of Tonverk is written in Rust, which has a much better toolchain as let’s say C/C++ or other older platforms. Testing framework is even built-in.
- The operating system of Tonverk is linux. Which means it’s possible to run and test (parts of the) software on your pc/computer in an emulation / docker container.
- Which also means automated testing is 100% in scope for all operations: You could write tests for them. Relatively “simple stuff” like copying and pasting trigs in the sequencer would have been caught by the testrunner which can run at any time in any CI environment, not just by testing on the machine itself.
But now I’m into "It should be simple to code" territory 
Either way, it’s easier to write tests for software nowadays so it should be easier to test too.