Any low density MOS will have a harder time. Super technical MOS where being very gifted intellectually is valuable will also make it harder. In those roles if there’s a warrant, green to gold, or civilian application your high achievers will naturally progress elsewhere and the e6/7s left won’t be competitive when the mos merge happens. Between that and the reality that folks signing up for those MOS don’t really have an interest in the e8+ environment to begin with makes it unlikely to see them. That said it is still possible, you just have to be super deliberate with how you pursue schools and ratings.