The only thing is the unknowns with the merger. Primarily whether US will stop honoring trials and if so when. While I think it's unlikely they wouldn't honor a trial that someone was in the middle of, it's one of the many questions that nobody knows the answer to. Both carriers have segment thresholds and US has them with the trial. Obviously the quicker you meet the requirements of the trial the better - once you've earned status it's unlikely that it would be taken away.
Jim