Michael Harvey's answer does give one possible dictionary definition of "financially risky", but not, I think, the one that best applies to this context.
When something is labeled "wildcat", it supposedly embodies some of the characteristics of these large felines. Wildcats are largely solitary animals that freely roam their territory, independent of social structures or attachments.
In a similar way a "wildcat" business will strike out on its own, often with limited (or no) backing from any outside investors, ignoring established rules (including safety guidelines), and hoping to achieve profitability before it a) runs out of money, b) runs afoul of some local authority, or c) has a catastrophic accident.
While these ventures are certainly risky the defining characteristic is independence. In theory, anyone can start up a "wildcat" business with a little bit of money and a lot of courage. In practice, few of these businesses succeed -- but as your passage says, some do eventually achieve huge success.
In this context, an "operator" is short for "an operator of a business" -- that is, the person who owns and runs the business.
