I suppose it depends on the day, but I have never had a problem on the SGRT or the LARIO (LA River/Rio Hondo) or SART. The SGRT has that necklace of parks for support (we like Liberty in Cerritos as a trailhead). The LARIO has fewer parks, but is a better trail from a design viewpoint (wider, flowering vines, etc.) .
There is a homeless encampment on the river a few miles above Long Beach Harbor. It does not seem to be a problem with riders.
Here is a map, photo set and survey information...
http://www.traillink.com/trail/lario-trail.aspx
If you check out dregerclock's channel on YouTube, you can find some vids of both trails and more and check them out at home.
Crime-wise the recent action has been on the SART. The same MO - pack of thugs toss beater bike to bring down rider - was popular on the Bellona Creek Trail some years back at the 405 underpass. Be alert of groups of louts and a beater bike ahead. I carry binoculars to glass the underpasses and trail when needed, but I ride a trike and can do that.
I would not be on any of these trails when the sun is setting. We are not talking Irvine here. The trails traverse regions of Not Good Addresses.