Having sat as a member on a pilot selection board, I can tell you that previous flying experience, while considered as part of an applicant's overall package, is actually fairly far down the list of factors considered by board members. ACS performance is up at the top, as CAPSS has been designed to bring to light an applicant's ability to adapt quickly to and comply fully with the required precision to fundamental elements (e.g. hdg, alt, a/s and changes to these) in a flying environment. The CAPSS tutorial booklet that the Recruiting Centres provide to PLT candidates is about the most valuable resource an applicant could read. Flying a flight simulator in IFR mode to mimic the sequences in the booklet would serve one well.
Imagine you're sitting on the selection board and you see the ACS results of two applicants, one with previous flying experience (say a PPL or CPL) and one without any experience. Say the previous flyer had a habit of sitting rock-steady at 100' above or below a target altitude and 5 kts below the target airspeed, while the inexperienced applicant was +/- 40' of the target altitude with occasional excursions to +/-100' and generally on-speed, +/- 3 kts, but sometimes varying +/- 5-8 kts, but trying hard to hit the numbers. Which of these applicants sounds like the person you think has the potential to fit well into the military?
Whether one had previous flying experience or not, the most important factor to success is attitude...have a positive one, striving to be as accurate as possible all the time, and you should be in good shape. Take things for granted and display an attitude of entitlement for whatever reason, don't expect to be around the flightline for long.
Anecdotally, I would say during my training overall, that those with PFE were no more likely to succeed than those without experience. In the extreme, during my Phase 2, only 11 out of 44 trainees passed and 8 of the 11 had no PFE...more than half of the 33 failures had PFE, several with CPLs and one I recall had an ATPL.
Regards
G2G