There are some LiteOn drives out considered to be rebadges of Optiarc drives (iHASx24 Y revision drives) or based on Optiarc design but with a little LiteOn-style programming included (iHASx24 W revision drives) that include NEC/Renesas chipsets. Outside of those drives, everything else has been MediaTek, including drives that were produced during the same span of time as these Y and W revision drives (A/B/C/D revision drives).
The firmware for these most recent MediaTek drives (A, B, C, D revisions) are different enough that flashing one over another will not work; there are other things that changed beyond chipset revision (and sometimes, in the case of going from C to D, the chipset remained the same while the internal design did not).