My question is whether malware in the embedded-controller firmware is something to worry about if the rest of the computer system is faithfully what it should be, together with standard security measures being in place.
Research & Instincts
My instincts are that it isn't something to worry about, due to the physical firmware-chip limits on the byte size of any such malware, and due to any such malware probably being incapable of communication with any bad actors or malware (whether over the internet or otherwise).
The Wikipedia page on embedded controllers does state that ECs can act as bridges between chipsets and the BIOS chips, so maybe EC malware could be capable of such bad communication and thus remote control from the outside world? It also says that they may use shared memory (probably the system memory) for communication, which might be another way that such bad communication could occur.
On the other hand, the Wikipedia page also says that embedded controllers have their own RAM, and are very 'deep' in the system; these things perhaps tend towards the view that EC malware isn't so much of a concern so long as the rest of the system is fine.
I'm trying to make my Acer C720 Chromebook computer safe to use, after deciding that it probably was compromised via a MITM-styled (interdiction-like) attack. I plan to re-install faithfully the system BIOS firmware, disconnect the WiFi+Bluetooth card in tandem with supplying the internet instead of through a new non-compromised dongle, and replace the system drive with a non-compromised new SSD drive.
For my particular situation, faithfully reinstalling the EC's firmware, or replacing the EC chip(s), might be quite difficult to do, which is why I've posted this question.