Question about shareholder class action lawsuits

SO's ancestor was one of the founders of the bank that later became Bancorpsouth, and she inherited a big block of stock, so I follow it pretty closely. BXS made a couple of acquisition deals in LA and TX last year. Over the summer, some compliance issues surfaced, and the acquisitions were put on hold. The stock lost about 20% in one day, and has since recovered some, but not all of that loss. Within a day's time, several lawfirms announced "investigations," and within a week had filed class action suits-I don't know what kind of investigation you can include in a week, but whatever. These sound like nuisance suits to get what they can out of BXS, but I don't really know much about how this sort of thing works.

TL;DR. How much of a threat are shareholder suits to a company? Do they usually succeed, or does the company pay a nominal amount to make them go away.