Just looked at the Dollywood site, and that disclaimer is still there. Just scroll to the Season Passholder Ride preview Days. It includes the verbiage "NightFlight Expedition will still be in technical rehearsal during preview days, and temporary downtime may occur. If the attraction is unavailable for your entire reserved timeslot, your preview ticket may be used at any point later that day."It was part of several news coverages of the official ride opening announcement. WVLT-8 mentioned this, as well as several other news outlets.
I see that upon opening, they had a two hour delay; but after that, most reviews I've seen are very positive. Don't know if the announcement is still up on Dollywood's site, but they had the disclaimer that this was being treated as a "dress rehearsal" and that not all elements would be available, as well as a disclaimer that ride issues may arise.
In the WVTL-8 coverage of the August opening announcement, they included similar verbiage, with a park rep stating the bit about ride elements.