I wouldn't be as worried about the charter company as about the safety of you, your crew and the boat. I recall a post about a charter where the single experienced person onboard was suddenly disabled and the boat sailed itself into the Atlantic because no one else onboard could take down the sails, start the engine, turnaround, etc. (He eventually came to and gave directions)

I have chartered with many newbies BUT ALWAYS with at least a first mate completely capable of taking the boat back to base.