What were their strengths and weaknesses?
A lack of tire was a big hindrance for the Spyders (and for the NAs and NBs, once the NC came along). The NC can fit either 275 or 285 (depending on the wheel:sidewall preference), while the others can maybe fit 255 while staying within the class rules. My car had coilovers, a lightweight flywheel, and a lighter exhaust (saved maybe 20 pounds), and the fat tires on light wheels in a class that allows changing seats, changing steering wheel, chassis bracing, swaybars, full intake and exhaust swaps, tuning, and more. The acceleration of the Spyders wasn't far behind that of the NC, but they couldn't maintain as much speed in the corners and they couldn't brake as effectively.
My car was about 180 crank hp with about 2500 pounds. A fully prepped NC would be 200 crank hp with about 2300 pounds.
The NC has fantastic brakes - the balance is wonderful and the ABS is absolutely perfect for competition. The ABS will allow the tire to reach its threshold of grip and slip just a little bit, and only interferes if the wheel is actually locking up. It can activate on only one wheel at a time, too, adjusting for brake bias issues that happen while cornering. I'm not sufficiently familiar with the Spyder's brakes to know how they compare, but the NC sets the standard high.
And the fat Hoosiers make it seem like the car can break the laws of physics in a corner. The NC's chassis is stiff for a convertible, and add in the huge grip from the Hoosiers and stiffer springs that can use the stiff chassis and huge grip, and it'll go through corners literally 20mph faster than it does on 225 width streets (a corner where my minimum speed is 40mph on the streets, the car will do at 60mph on fat Hoosiers).
The fat tires are an autocross specific setup, and slow the car down on road courses. On road courses, 225 is about as wide as a naturally aspirated NC should go (barring an engine swap). Fatter and acceleration is badly impacted over about 65 to 70mph.
~1,600 pounds. With its late '50s chassis, anemic engine, and rear leaf springs it is unmatched by today's cars.
GrandPa class (GP) is calling!