I represented my airline about 25 times in small claims court (they usually send a local manager who has been briefed by an attorney versus paying for an attorney to attend). Small claims is unpredictable. I have lost for damages/missed work because somebody missed their flight... a delay was posted of like 3 hours early in the day, then closer to departure time, an airplane swap enabled it to leave on time. Two out of 150 missed the flight. They won because the judge said "I have never heard of a delayed flight getting put back on time." Another time, won because a passenger claimed injury from their seat assignment (non-reclining)... pointed out contract of carriage advises if specific seating is needed for medical reasons, must notify carrier. Plus, don't dispute the passenger's medical bills or pain, but they do not have a legal right to sue for it - it was a flight to Mexico thus governed by Montreal Convention that states you cannot claim injury on an international flight if an incident/accident has not been recorded for that flight (under specific specifications), and we have none such record therefore sorry but not allowed to file suit.

Most small claims/district court judges have just as hard a time with Timatic, contract of carriage, etc. My advice would be to spend the few hundred bucks to get an attorney to go with you. United's contract of carriage (I just did a quick search of it) contains such vagaries like they can deny boarding for international flights and not be held liable if: "The government required travel documents of such Passenger appear not to be in order according to UA's reasonable belief;" What constitutes reasonable belief can be a good argument. And it also specifies a process for complaining and steps to work to resolve with them before resorting to a lawsuit, otherwise per the CoC you agree United can sue you to collect its attorney fees.

It is worth a shot. The filing fees are usually pretty low but it can be drawn out and could spend a whole day. But there are simpler cases where it's so glaringly obvious the customer messed up, and they win. But I would advise looking up to at least consult with an attorney who is familiar with the process in whatever jurisdiction and has dealt with airline suits before. The only ones I have seen side with the airline very predictably are those involving checked baggage. Every judge seems to comprehend it isn't a good idea, or likely fraud, to put $10k in cash in a checked unlocked bag then claim it got stolen.