The rules will be different for every contest so we need to make sure that we understand and will not break. We know what will happen if we break one of the rules so we will always be careful selecting the contests. We don't want our winning nullified by the casinos because of the mistake.
If everything is fine, you can withdraw your win money without a problem. You may search for the other contests that you can fill the requirements and leave the hard requirements. You don't want to feel difficult because of the rules.
If a same company and same manager run contests, rules can be not too different while there is always possible chances if the company and manager see needs of changing contest rules for better effects.
If it a same company but contests hosted by different managers, you will have higher chances of many different terms. People as possible participants must read terms very carefully before joining if they don't want to miss important information. They can be disqualified by ignoring contest rules and this possibly causes initial fund loss by lack of carefulness for Reading before Joining.