Flies may carry parasites and can cause gastrointestinal issues if consumed in large quantities.
Additionally, fly bites on ears can be a problem for outdoor cats.
Suggestions
It's best to prevent your cat from eating flies.
If your cat shows signs of gastrointestinal distress or if you notice fly bites, consult your veterinarian.
Note: even if labeled as safe, any food can potentially cause allergies. When introducing a new food, please feed a small amount and observe your cat for any allergic reactions.