If Apple wants to add some brand new feature to Mach-O, they generally just define a new load command. There's generally just one way to do it. The main downside, is (in practice) only Apple can do it. [0]

Whereas, with ELF – if you want to add a new feature, do you add it as a new program table entry type (PT_), or a new note type (NT_), or a separate note section (SHT_NOTE)?

[0] Well, historically OSF/1 also used Mach-O, although my understanding is it was an incompatible implementation of the same basic ideas. The main vendor to ship an OSF/1-based Unix was DEC (later Compaq then HP Tru64 Unix), although HP also (briefly) shipped their own OSF/1-based Unix (prior to buying DEC/Compaq), as did IBM (the short-lived AIX/ESA for IBM mainframes, which despite its name, had very little in common with the AIX everyone knows), and also various supercomputer vendors (Intel Paragon and ASCI Red, Hitachi, Kendall Square Research). But all those systems are defunct, so Apple's Mach-O is the only member of the "Mach-O family" still in production use.