One concern is that they are slower swimmers so more vulnerable to prey.

I have an Oranda that's two years old now and the fattest goldfish in the pond. A smaller Ryukin has also done fine.

The rest are comets (well, one is a Sarasa). No issue at all with them this year. Last year I did lose two Ryukins during a very wet spring when my pond flooded over. I think they were rooting in the new expanded shallows and got snatched by a bird or raccoon.

I had read fan tails didn't survive winter like the others, but haven't tried it.

We had one of the harshest winters last year that I can remember, and both my fan tails survived fine.

A friend has had a fantail for a few years left out year round and she (the fish) is doing fine. Friend is doing pretty good too. LOL