Air China is no better or worse than most airlines in Y. Your in flight experience should be fine.

However, transiting in Beijing can be a pain. I would leave at least 4 hours in case of ATC delays, where a 2 hour delay is normal. Add to the fact that transit immigration and security is horribly inefficient. Since you're transiting from CA to Air Koryo, you will need to go from T3 to T2. There's no airside connection, so you will need to clear immigration, reclaim your bags (if any), and clear customs in T3. Then take a shuttle bus to T2. You need to go through check-in, immigration, and security all over again in T2.

My dad took IAH-SHA via PEK last week. They made him weigh the carry on at check-in. 5 kg was enforced, and if you're over, the bag will be checked.

I agree that CA is awful with IRROPS handling. Just allow 4+ hours to connect and hope nothing happens. Good luck!