I don't have all details, but seems like lots of work:

* Mach APIs instead of ptrace (probably a lot of changes).

* Mach-O instead of ELF.

* Some other APIs instead of /proc/<pid>/{maps,stat,...}

* Probably arm in addition to x86?

* Dealing with security stuff.

* Probably lots of other small differences everywhere.

Limiting the scope to one OS and CPU arhitecture was a big part of how I was able to make a usable debugger in a reasonable time.