Although many factors go into the evaluation of a potential class action from the plaintiffs' perspective, here is an important one: If the case is viable, you will have a good idea at the beginning of a case how you'll be able to fashion a settlement at the end.
Some potential class actions are just not possible to settle. Perhaps the cost to administer the settlement would greatly exceed the benefits to the class. Maybe the defendant, if it agreed to settle, would have to cease doing business. Or perhaps the case is so unusual that it does not lend itself to one of the common settlement models.
Wise class-action lawyers do not embark on new litigation without a clear strategy for the end game. Not only is it the best way to proceed from a business perspective, it's also good lawyering. After all, if problems with the case create too many barriers to settlement, your opponent will argue at the class-certification stage that the case is unmanageable as a class action. If the problems really exist, he's likely to prevail.