Assuming you're asking/looking for a budget/entry level board for unlocking, i have used and tested three model, ASROCK N68-S3 UCC, ASROCK N68-VS3 UCC and ASUS M5A78L-MLX
and what i can say is ASROCK N68-S3 is the best out of three. i was able to unlock a phenomII x2 555BE 3.2GHz into fully working and stable quad-core b55 using all three board, but only ASROCK N68-S3 was able to achieve 3.7GHz stable running 30minutes prime95 (air-conditioned room). the other two board was only able to reach 3.6 (but that is only after increasing the vcore).

as far as undervoltage goes, none of the three capable of intelligently undervolt. meaning that the voltage stays at the value i set, be it when cool n' quiet activate to slows down the processor or at full speed. from my experience, this doesn't happen on higher-end boards.