California is a big state with lots of different climates, the weather in the Central Valley pretty much sucks, yet lots of really good runners come from there and parts of it have a great running scene.
Even LA has really different areas, if you are in the beach communities, say Manhattan or Hermosa the weather is great for training year round, but trail options are limited and the good trails in the hills to the north are a pretty long drive through traffic. Eastern parts of LA get a lot of hot weather and don't have many good places to run.
San Diego has good weather at the beach for training, but the trails are in much hotter areas, good for winter training for sure.
Norther California has some great trails and the spring, summer and fall weather are good for training, but the winters are wet.
The Sierras have incredible trials and altitude, but obviously don't work year round.
If you could move around California through the year to follow the best weather, then you could train on great trails and mix some altitude in. So overall, I don't think California can be beat for training.