Delays on the Punta Arenas to King George Island run are common enough that you should plan for the possibility, and roughly 3 percent of fly departures cancel outright in a given year. The runway is a gravel strip with no instrument landing, so the weather has to cooperate at both ends. Operators handle cancellations differently, so the specific terms are something to check on the departure you're considering rather than assume.
This is the main argument for sailing if you're undecided. A weather delay on a sailed voyage usually costs you a few hours of schedule. A weather delay on a fly voyage can cost you days of a trip that's already short.
Because the handling varies by operator, we always pull the actual policy for your departure before you commit. Ask us and we'll get it in writing rather than paraphrasing it.