Despite very limited usage, metal straws have caused major injuries including fatalities. Turns out having a metal stick pointed at all sorts of sensitive soft tissue is a risk.
Meanwhile, if using your own straw with a restaurants disposable cup, hardly helps since the cup is still being waste. If using it with reusable cups, it won’t save you from any sanitation issues, since the drink is right in contact with the container. It may be useful for sanitation reasons with a can, but again, the can is disposable. Even if you recycle it, the coating on the inside and the paint on the outside probably are about as much as the plastic straw you spared.