All other considerations aside, there is one thing in patch 8.3 that Blizzard would NEVER put live without the most thorough testing:
The new Auction house.
If your mount glitches, your mount glitches
If your raid boss has cheese, you hotfix it next week and a week of free loot goes out
If the auction house has a repeatable exploit, you can create damage to the in-game economy so severe you might have to do rollbacks. It would be one of the biggest disasters in WoW history.