I eat Daikon's regularly. They have a much milder flavor. They're not spicy at all, and have a soft turnip sort of flavor. I use them in place of potatoes in some dishes since they're so mild. They're also delicious pickled.
I'm not sure about the flea beetles. Sorry! I do know cabbage maggots will eat them.
It would really surprise me if the flea beetles didn't go after the daikon radish.
They chew on all the brassicas and especially like the more common radish.
The way I have eaten daikon is in Vietnamese sandwiches. I tried a daikon variety in the garden that was supposed to be for use a greens crop. Planting early, planting late - it didn't work. With little leaf growth, the plants too quickly bolted to seed ... so I had some seed pods to enjoy. They were growing in the home garden where flea beetles are not much of a problem. In the big veggie garden - they are out there in big numbers every year. Perhaps, because of the wild mustard and alfalfa which is available even if they don't happen onto my garden plants.