That's really irrelevant for getting native Linux running optimally.
The stumbling block is creating good reverse engineered drivers for all the custom hardware, or you end up leaving a lot of functionality behind.
It depends on what the real goal of this is. I think most of these people are doing it for intellectual curiousity, not trying to produce the most performant possible Linux PC. Is it really necessary to have maximally accelerated GPU drivers, for instance? Or take advantage of every method Apple provides for CPU power saving? They may be able to reverse engineer some of that, while other bits remain a mystery, and end up with something highly usable that falls a bit short of matching the full performance and efficiency of the same Mac running macOS. If it is 90 or 95% of the way there, I don't think anyone would consider the effort to be less than a rousing success.
It sounds like at least some of the undocumented Apple exclusive hardware (aside from the CPU & GPU) may not be changing much year to year, so they won't be wasting time reverse engineering e.g. a new uART every year. Stuff like wifi and bluetooth come from Broadcom and is presumably the same as or similar enough to Broadcom devices that have Linux drivers that they aren't any problem. I wouldn't be surprised if they license IP blocks for stuff like PCIe and USB rather than roll their own, if so there will be Linux drivers to be had for those as well. So you probably don't leave much functionality behind, just a certain (hopefully modest) percentage of performance and power efficiency.
It just isn't going to be worth the effort to wring every last bit of performance and efficiency out of the CPU and GPU. The 80/20 or 90/10 law would probably make that a real slog and then you likely have to do it all over again every year or two. Maybe the people doing this will have their interest piqued long enough or be able to derive enough information from examining macOS drivers to reach 100%, but if they don't that's fine.