In the "spinach gratin" recipe what is the downside in using fresh spinach?

Frozen food is usually frozen immediately after harvest, whereas fresh produce often sits on the shelf, losing some of its flavor. Fresh spinach is also a lot more work to prepare. You can definitely substitute fresh, if that's what you have on hand. Just be sure to squeeze it very well after cooking, to release the excess water, before adding it to the casserole. If you're going shopping, go ahead & buy the frozen.

The main downside of using fresh spinach is that you'll have to buy a ton of it. Using frozen spinach is a way of saving money.