Frontier Airlines is pushing the limits of consumer complaints with nearly 50 complaints per 100,000 boardings. That's an eye-popping 5,523 complaints. Hawaiian, at number two, was a victim of a strict Hawaiian quarantine rule. And then there's United Airlines, with the most complaints of any U.S. carrier — 11,274.

Several international airlines that refused to offer prompt refunds. They include:

  • Air Canada (5,784 complaints)
  • El Al (2,245 complaints)
  • Lufthansa (2,122 complaints)
  • TAP Air Portugal (5,007 complaints)