I've taken on a few nutty set building projects...
Lord Stanley's Mug I loved because it was difficult but achievable - at least excluding 1/1's, there are 392 tough cards (x/9 or x/10), plus another 120 x/9's from the Expo-exclusive Decades set. That really felt like the perfect sweet spot of a real challenge, without ever feeling impossible. Even if a few copies disappeared into PC's, and a couple sit on the shelf unopened, I was confident I'd be able to complete it eventually.
I waffled for quite some time on my latest project - ITGU Vintage Memorabilia... would've been a no-brainer if there were no parallels, an average of 15 copies of each card and ranging from 8 - 24 copies would be challenging but fairly doable. With six levels of parallels though, I thought long and hard about whether to attempt the most common (silver) level with an average of 6 copies per card but a handful of x/2 and x/3's... in the end I fell in love with the design so I'm biting the bullet and taking a stab at it, though I really don't know if I will ever complete it as very few new copies are surfacing and I'm still two dozen cards away from the 88 card set.
I don't think I'll attempt another set quite so crazy ever again, if I ever get tempted to build another set it'd most likely be something in the x/10 - x/25 range, with perhaps a handful of SP x/5.