The short answer is that this is a different beast. Much of the hardware in Macs before the M1 was commodity hardware. Now it is all bespoke and undocumented. This is a big undertaking.
But yeah, short-term there's a bit of a barrier to entry. Once it's overcome, though, this might be what pushes me to voluntarily buy any piece of Apple hardware made after 2007.
I am not sure why would would expect better success with this processor than we have had with x86? Most PC hardware is directly supported by the OEM in Linux.