Storage. Hard drives and SSDs.
Since 2018 and the T2 chip, the BIOS is stored on the soldered storage chip pair. Either one of those two chips or a capacitor adjacent to them dies, you've not only lost your files and OS. You can't replace them. You can't even plug in an external drive to boot off of since the BIOS is now stored on the SSD chips as well instead of a separate ROM as before.
2016-2017 soldered the storage, but you could at least work around it. 2018+, there are no more workarounds. If any part of your SSD fails now, you've got an expensive useless brick.
Swapping wear your SSD out faster? SOL. Capacitor fails and shunts 13V through your SSD? SOL. Anything at all happens to either one of your SSD chips? SOL.
Either pay for that AppleCare in perpetuity or buy a whole new laptop. There are no more workarounds.