Switching HCM mid-year — is January 1 actually the only sensible cutover date?

Common wisdom is migrate Jan 1 so W-2 reporting lives entirely on one system. We're under pressure to switch in Q3 because our current platform is failing badly.

For anyone who's done a mid-year HCM migration: how did the W-2 + tax filing split work in practice? Was the operational pain worth not waiting for January? What did you bring over vs. leave behind on the old system?