So here are summarized findings for enum and int types in the new switch expressions and statements:

Incompleteness checks for Enum types:
switch expression: static error message
switch statement: static warning, no runtime checks, Fall-Thru semantics for missing cases!

Incompleteness checks for int types:
switch expression: static error message (asking for default case)
switch statement: NO warning, no runtime checks, Fall-Thru semantics for missing cases!

The general claim for completeness checks is not borne out.

Incompleteness checks for (sealed) classes:
still need to take a look at Sean's code ....