At UND there were were very few restrictions on the weather minimums we could fly in. I have shot approaches with students at airports where the weather was reported as "less than 001OVC and less than 1/4mi vis" knowing we wouldn't see the runway. Good experience for a student to actually shoot an approach heads up and not see anything and go missed. The few restrictions we had were no simulated instrument or powerplant failures in IMC and at night you had to add 100' and 1/2mi to the minimums required for the approach. The experience is invaluable to students and some of the most worthwhile time they can spend in an airplane.