There are a very small number of credit unions currently offering mortgages, as they do so individually, and only a few are big enough. The only way they'll ever be able to offer anything of scale is if they can pool their resources, and offer centrally, which would require legislative changes, and move away from the common bond ethos of the CU movement which might backfire.
Any pricing I've seen isn't cheaper than what's available for those that can switch, but less than some SVR rates. Mid 3% range.
There is however big demand for them. I'm aware of 1 CU that has a waiting list.