Yes, Kayak Hacker Fares are safe as long as you understand how they work. Hacker Fares are simply a combination of one-way flights that are cheaper than a round-trip. These flights can be on separate airlines, which means the second leg of your flight might not be protected.
For instance, if bad weather cancels your outbound flight, you might decide to stay home and skip your trip altogether. But since your return flight is on a separate reservation, it will not be refunded like your canceled outbound flight. Overall, this risk is relatively minor, but it’s good to know before you purchase a Hacker Fare.
If you are booking a Hacker Stay for your hotel, this will consist of 2 separate reservations. The only downside to this is that you might have to switch rooms during your stay. However, many times you can ask the front desk to combine your reservations when you’re checking in.