no, plastic is generally safer. One of the most important safety innovation in cars is the plastic gas tank (as a side note, we are looking for something as transformative for batteries in electric cars), it bends way further than steel before leaking.
Normal trucks are not stupid, it's just that past a certain size, only steel can hold the static force of the liquid. There are contradictory factors, you want the cargo to be held firmly in place when you brake, but you want the tank to be accepting a lot of deformation before breaking. I'm not sure, but a secondary concern might be contamination, I'm not sure you can switch liquids in a plastic tank, while you can clean a steel tank.
Bigger tanks also flex more. So they need something that won't fatigue as quickly. If it spends its life bouncing down roads, plastic will begin to show cracks at flex points long before steal.
Normal trucks are not stupid, it's just that past a certain size, only steel can hold the static force of the liquid. There are contradictory factors, you want the cargo to be held firmly in place when you brake, but you want the tank to be accepting a lot of deformation before breaking. I'm not sure, but a secondary concern might be contamination, I'm not sure you can switch liquids in a plastic tank, while you can clean a steel tank.