I don't know how it in Spain (but I heard their rail system is good), but in my country it's the opposite. Trains sometimes have small-ish delays of 5-60 minutes, but longer delays are rare and there are usually other trains running the same route, so you can easily switch (for free if it's the same company). I've never experienced a cancelled train.

With airplane, you often get these small delays as well, but there's also the risk of a very long delay. I had the pleasure of waiting 12 hours for a (connecting) 50 minute flight.