Interesting isn't it, I've seen both NGC 7006 & 6717 through a 4.5" f8 reflector at low powers (43x) several times in excellent dark skies. To me they don't seem terribly challenging objects compared to some of the much fainter ones I've chased down with that set-up.
I'm not a great one for observing notes but I did find these sparse entries from 2007:
'
NGC 6717 (Sagittarius): said to be “challenging object even for large aperture scope” – relatively easy in 4˝”. Very tiny fuzzy glow beside bright star, averted vision not needed. Seen 16 June 2007.'
'
NGC 7006 (Delphinus): very remote GC. Easy to see in 4.5”. Dim, very small, centrally brightened glow. Seen 5 September 2007.'
With 6717, maybe the larger apertures turn Nu Sag into a blinding glare, inhibiting the view of such a small subtle object.
Cheers -