I'd agree the AMT Avanti is a tricky one. I love Studes too. I've been building for 40 years, and I find I have to distinguish between "tricky" and "well-nigh impossible"
The old AMT kits with opening doors and such can be tricky, but the problems with the doors can usually be solved by careful fitting and trimming before painting and asemby - says he who hardly ever does this!! I love the old '56 Ford kit - I've built about eight - though now that I think about it the doors usually don't fit properly, but as I never show my models that's never bothered me.
My unicorns would have to be kits which have taken 20+ years to build. No, not Pochers (never tried one!); kits that I've got stuck on, and put away in disgust, to revisit them years later. So what fits into that category, you ask?
Well, there was the Otaki 1/20 scale Datsun 240Z (Fairlady Z432) from the seventies. I started it before I was married - we celebrated out 24th anniversary last month. It's still not finished, but only needs some BMF now. This one was a unicorn for several reasons. Firstly, the electrics. It's motorised for forward and reverse, as well as working headlights. Trying to feed those wires front to back where they wouldn't show was no fun. I think everything will work, but I have no intention of putting batteries in it to drive the fruit of my not-inconsiderable labour. Then there were the seats. Working reclining seats sound great, but there's no friction in the mechanism, so they instantly fallback to the fully-reclined position. I stuffed up the paint - but that's not the kit's fault. And then, when I got it all together, I find you can see the ground through the side windows - despite the opening doors, there are half-inch gaps between the firewall and the centre console, and between the battery box under the hatch and the side of the body. UNICORN!
Then there was the old 1/24 Pyro VW Beetle. It sorta,kinda went together OK, but despite the opening trunk with luggage, there were no trunk side panels so you could see the wheels as soon as you opened the trunk. Same in back with the engine.
My biggest unicorn at the moment is more like a rhinoceros. Well, mentally, anyway. It's the Heller 1/16 scale Citroen DS. Has anyone else tried this? It has a lot of things right, until you go to install the body on the chassis - and find the rear seat and parcel shelf doesn't want to tuck up into that tight tapered tail. Unicorn? Yep - 30 years and counting.....