Cadence is a buggy program. This being said, it's all about the models. I've designed some pretty "wild" (weak inversion, UBER low voltage, nA) type circuits and they were very close to the models, and therefore the simulations.
It's all about the models for accuracy. Have a good kit designed for Cadence, you'll get wicked results. Have a bad kit, you are in trouble.
That being said, I've read H-Spice is more accurate with proper models than Spectre, but with BSIM3.3 and Spectre, you can get quite awesome simulations.