It is not often down, but it's very easy to hit its rate limiter when upvoting or commenting. Waiting a minute usually fixes it.

The rate limiter is even stricter when not logged in — for a period I wanted to take a break with getting into online arguments, so I stayed logged out, which would cause my IP to be banned after a day or two (a bit annoying, though easily unbanned with the self-service system: https://news.ycombinator.com/item?id=4761102) — IIRC dang confirmed that it would be better to stay logged in to avoid this anti-bot measure.

I guess that's the price to pay to have such a popular website that's maintained by a single person.