quoted

Could any of the more knowledgeable on here explain why
is it such a bad idea please?
    

Because there's no handshaking between the firmware and the OS driver, 
and accesses to hardware sensors are often indexed. Imagine this 
scenario:

ACPI                             lmsensors
----                             ---------
select temperature register
                                 select fan speed register
read value
                                 read value

ACPi will read the fan speed register instead of the temperature 
register, and the value may be far too high and cause a critical 
shutdown of the system.

That's the *good* outcome. The bad outcome involves these register 
accesses racing in a way that disables fan control or thermal trip 
points and risks causing hardware damage. It's not safe for two 
different codepaths to access the same hardware without having any kind 
of locking, so if your system firmware declares that ACPI is using the 
temperature device the hardware sensors framework will refuse to.
  

All noted, thanks for the explanation - pretty hairy stuff! The tragic 
thing is, I have no way of telling ACPI to *not* use or "implement" its 
own fan, voltage, temperature management and let a more capable piece of 
software (the f71882fg driver in this case) do that job!

What is the alternative? There is none that I could see. I tried using 
memmap to force ACPI not to use the memory region claimed by f71882fg 
(0x290 - 8 bytes long according to the driver), but that didn't help 
much. What am I supposed to do - deactivate ACPI completely? That would 
be like going after a fly with a bazooka!