Changing Rigidbody drag on the fly

Is there a problem with dynamically changing the drag of a rigidbody? (For example, to slow down an object if it is above a given velocity.) Are there situations where changing it too frequently (such as in every FixedUpdate call) could result in performance issues?

No, I don't think there are. This is a useful technique and one which has many practical applications.

For example, I have used code which dynamically changes the drag of my car physics object, depending on how deep it is submerged in water.