One is like Intel ME. It supports use cases you as an end user do not give one single damn about and could easily do without, if given the option. Clean and remove as much as possible with no remorse.
The other is like SMM. Believe it or not, it's only accidentally the intrusive and insecure monster it has become. The point was exactly what it says on the tin. System Management Mode. The OS would ask the 'system' to do something it didn't know how to do, like change some power configuration in a laptop, and the firmware would handle it and then give control back. But these operations were delicate and needed to not be interrupted. And the hardware involved was delicate and needed to not be touched in the wrong way, lest the system hang or even fry. So it was locked away where the OS couldn't touch it. And then people started noticing they could use an untouchable special execution mode in other ways, and, well, here we are. The unfortunate thing is that because of how it started, you would feel some pain on most systems today trying to get rid of it. Your OS does not have drivers to change the CPU thermal characteristics properly and in accordance with silicon design (because the vendor did not develop one or make the information available to the outside world). So removing SMM will make many 'nice' features stop working. You may lose the ability to suspend and resume. Your power draw may be stuck too high or too low. It's possible on some systems you would be fine, but on others not so much.
Like the people demanding that graphics vendors provide open source drivers rather than binary blobs in the hope of making things better, really what's needed here is advocates pushing for the tasks performed in SMM today to be migrated to OS drivers, and from there to open source drivers.
You as a single tinker today aren't likely to get all this stuff working with the non-existent documentation.
Blame the silicon vendor for doing things in the most scary, back-door-ish way possible. UEFI didn't make them do that. The same crappy business practices that drive all bad proprietary software decisions did.