On the minimum safety thing you can be sure -- if they're based in a well-regulated jurisdiction with honest enforcement such as Britain or Canada.

Your question about how they stay in business is answered by their apparently whimsical cancellation of under-sold flights. For a cheapie airline, it's crucial to put a bum in every single seat on every single plane that takes off, and to serve only destinations where so many people want to go that full planes are the norm.

For a demonstration of this principle, compare flights between Toronto and Halifax with flights between Toronto and, say, the Bahamas. In February.