Freeze dried tubifex are quite messy, but i have been known to feed them a few times a year to most of my fish. There is a risk of transmitting bacteria to your freshwater fish with these (and other freeze dried foods that come from freshwater such as bloodworms). These types of harmful bacteria are not killed off by being frozen or dried. However to get around this, many brands sterilize their freeze dried making them perfectly safe and frozen foods, hikari is one well known brand that does this.
Freeze dried foods in small amounts every once in a while is great for a treat, but frozen stuff is much preferable as it is usually more palatable for the fish, and it has less risk of constipating the fish. Especially for fish prone to constipation such as balloon mollys, bettas, fancy goldfish, and blood parrots.