Power adapter efficiency standards, any drawbacks?
The effect will vary based on prospective.


Power Supply Manufacturers: Major - All products will have to be updated and qualified to meet the new standards, new part numbers created, etc.
Distributors: Moderate - All old stock must be sold off and replaced, part numbers for old supplies obsoleted, and replaced with new, primarily paperwork.
OEMs: Minimal - Use existing stock, update purchased part number.
End users: None - Won't even notice, except maybe the $0.01 difference in utility bills...


The old adapters won't go away. People will still use them until they fail. There won't be significant difference in circuitry or cost from old to new. 

At the end of the day, these directives are pretty dumb. Who cares about minimizing fractions of a Watt of no load power when people are using electricity to heat and cool their homes, as well as refrigerate food and charge electric cars?