Hard/soft does make a difference in how the bullet impacting reacts. But, it is not a factor in safety.
All that matters is that the targets are supported in a manner that does not direct spatter back towards the shooter.
Standing targets should have bases only heavy enough to hold the target upright.
Swinging targets must swing easily.
Hanging targets must hang freely so as to allow spatter to go downwards.
I used to manufacture steel sillhouette targets and learned some lessons the hard way.
My rule is to never shoot steel closer than 25 yards. But, I have broken that rule many times with swinging mano-o-mano dueling trees. As long as the plate swings the spatter will go to the side.
Still do take precautions, safety glasses are a must.
Type of steel is usualy determined by local availability and your budget.
Most steels at 3/8" will hold up fairly well until someone shows up with loaded to the max-max 300 gr bullet in a .44 mag. or a Causull, etc.
1/2" hard face will last a long time.