There's nothing unsupported about AHCI. Some BIOS's have a USB handback issue which people seem to associate with AHCI mode, but really the two are unconnected as far as EEPC is concerned.
The BIOS may show the bug more in AHCI mode though, that's conceivable.
That is correct. Those two issues are currently most important for EEPC 5.x:
- disk access errors (especially in pre-boot)
- USB handling in pre-boot
Both are only partially resolved and poorly documented (as to what model PC's they affect).
So it is for customer diligence to test before use.
A step back, compare with SafeBoot 4.2.