I’ve heard urine and feces are a big problem, on Bay Area Rapid Transit trains, in the SF Bay area.
I’ve ridden it many times when I rented a car at the OAK airport, then returned it later.
I am wary of riding the BART at night. But it’s so much cheaper than an Uber (about 35 vs. 11.10 for the BART from OAK airport to Pleasanton) I prefer the BART in the daytime.
My reasons for not fully trusting the BART? See here (back in 2017, but crime is only worse in the SFO/OAK area) — should have been national news, but wasn’t:
https://www.sfgate.com/crime/article/BART-takeover-robbery-50-to-60-teens-swarm-11094745.php