This is the exact issue. When I saw the forecast for Sunday and Monday, I thought it was a joke. I moved here in 1996. We've never had a drop of rain during August since I've lived here. We've also never had the amount of rain that's currently forecast and we have had significant flooding problems across LA since 1996. The flooding would not only be a problem for transportation, but it is possibly going to disrupt power, water, sewage, cable, internet and other infrastructure based services. LAX might even have to shut down temporarily.
If I lived on the ground floor of my apartment building, I'd be concerned about getting a few inches of water in my apartment. If I parked in a below surface level garage, which most apartments and buildings have, I'd be worried about my car ending up in a concrete pond/swimming pool. They are doing work on the sewage lines at the end of my block, so yeah, that's probably going to be a problem.
The other thing is that roofs in Los Angeles don't get these kinds of rain. I expect a lot of buildings to have roof leaks, not from winds damaging the roof, just from too much rain too fast for the roofs. I'm bringing my tarp into my apartment just in case I need to cover my computer and television.
I feel empathy for everyone in Southern CA who lives on a hillside/mountain, especially in burn areas.
Unless someone is foolish, it shouldn't be a life threatening storm, but if it drops as much rain as they are forecasting, it's going to do a lot of damage to infrastructure. It would really hurt and leave a lasting mark in that regards.