There’s also issues with bishul akum here too – as well as ingredients, Jews milking cows on shabbos, and using the milk in production.
With restaurants there’s a huge issue of supervision. As AvireDeArah said, terumah, maaser, smittah – even with a good hashgocha if there’s not supervision somewhere what happens when they run out of cucumbers? Any joe shmoe who isn’t frum will go to any store to pick up more.
A woman in the north died this past year after eating milchig ice cream in a meat rabbanut restaurant.