Some KML viewers will incorrectly display features if they have more than 30,000 vertices. If the original data had more than 30,000 vertices in a single feature, you may experience rendering issues in the KML client.
If the KML client is experiencing issues with rendering a particular feature, you can use a different KML client.
Alternatively, you can modify the original data before converting it to KML. The Split tool or Generalize tool can be used to simplify your data.