I live in Northern California, but visit in-laws in Costa Mesa twice a year, and on each visit I usually have an early morning ride from Costa Mesa to Green River and back. I rode two days ago on July 4.
The stretch from Chapman to Taft was certainly different than it was when I last rode, late last December. I'm not sure, however, that it felt more dangerous, even if it seemed crazier. There really weren't encampments in the underpasses, and that is what had bugged me the most in the past. In the underpasses, there is little room, and too many things, whether belongings, bikes, or dogs, seem to end up on the trail. No way to have any speed going down, not when there is a danger of anything being on the trail combined with going from light into dark.
The encampments on both sides of the trail out in the light didn't seem so dangerous, even if it seems like the number of people camped out there has exploded absurdly. It didn't seem unsanitary to simply cruise through the area. It didn't seem unsafe. Of course, between 730 a.m. and 10:30 a.m. on the 4th of July is probably busy enough so there are no problems, it may be a different story when bike traffic is light. The only thing that was really in the trail during my ride through the homeless stretch was one medium size dog sleeping with his body just off the trail and his head in it, and otherwise, there were really no worries on this ride.
The only area where there were tents under any roads was at the 57, but they were just on the other side of the wall towards the river from the path, so there wasn't a safety issue presented under the 57.
Do I feel as safe as I would if there were no homeless encampments? No. But the SART doesn't feel as dangerous as, say, the PCH, or riding Adams from Huntington Beach back towards my in-laws' house -- last Sunday, I had one jerk who wanted me to take a right turn lane instead of staying on the right side of the far right lane that was going straight (and I could see in my mirror a car behind me wanting to turn right). He honked as he passed close by, and after getting through the intersection, he slowed down for a confrontation. Seeing this, I stopped at the corner after getting through the intersection -- since I wasn't going to come to him, and he couldn't really back up, he gave up and went on his way. I can't say that the homeless make me feel comfortable, but I feel way more threatened by idiot drivers than I do by the homeless.