The statement by USCIS apparently means that there is some reason - often security-related -why it will not be acting on your petition within the 15 day premium processing window. It doesn't necessarily mean that your I-129 will be rejected (the vast majority of security related delays are eventually resolved without a denial). Meanwhile, you can - and probably should - apply to extend your OPT (if you are eligible to do so) right up until your status is changed to H-1B. There's no harm in applying to extend your OPT while a change of status to H-1B is pending. It simply will become moot if the change of status is approved.