Speaking from first hand experience, the worst thing about it is time and money to defend the suit. I and a couple of others were sued for $5,000,000 for leaving one organization and "competing" against the first. We were all board members and disagreed with the direction that the first program was heading.
After thousands of dollars of legal fees (not sure where they got their money but our legal fees were paid for by one of the individuals personal account), it was settled without a dime being paid to them by our organization but we did have a "who is running what league during what season" agreement.
Both programs are 501c3 and they tried to get a restraining order to shut us down completely until the case was settled. The judge ruled in our favor and then the serious settlement talks made some progress.
Anyone can sue anyone else... there is a cost to defend that can add up into the thousands of dollars.