Get some time in - why should this adversely impact on NCA in the way you are referring. Note that the Defence Board have repeatedly stated that the new NEM offer is about recruitment and retention.
By my reckoning, and averaging out promotions during a 42 year career, an NEM NCA would spend 9 years marking time as a Sgt, and 11 years marking time as a FS. Present this to people and in my eyes there would be a massive recruitment and retention issue.
Not to forget there is no good news about flying pay banding around...
18 - Join
20 - Sgt Level 1
24 - Sgt 5
33 - FS 1 (Promotion)
37 - FS 5
48 - MACR 1 (Promotion)
52 - MACR 5 (Max pay band)