I watched someone wash out of a level 5 up/down, and I sincerely believe that they didn't make it due to lack of consistent traffic and training opportunities (the facility was extremely seasonal). When the place got going, it was very complex and could kick your butt, but that was rare and you had to be able to handle it even if you hadn't seen it in six months. I have since worked at a level 6 VFR tower with incredibly consistent traffic and training opportunities, and I truly believe that someone with no FAA experience has a far better chance making it at a low level VFR tower with consistent numbers than at a low level up/down with sporadic traffic. This is why I believe that if someone shows potential and a good work ethic, they should be given a second chance regardless of their current facility level. Some people are really good at VFR towers, others excel in a radar environment, IFR towers, etc.
I've also seen someone who was a CPC-IT end up through the NEST and get screwed pretty badly. I don't think the NEST should apply to CPC-ITs. It makes it extremely unappealing for people with a few years' experience to try to move up if they may get sent to the far lands of the FAA if they struggle. Like someone above said, if you know you're standing a chance of not making it, your best bet it to try to ERR before they terminate training.