safety is a must you are dealing with fire electricity.
should have eye protection the arc can blind you like staring into the sun. full body coverings as light from the arc can give you burns there is also Extreme heat, arc is at around 9000 degrees.
if you are using shielding gas (not flux cored) work needs to be done inside because wind will affect weld quality so ventilation is required. also the shielding gas is highly compresses so good regulators and knowledge is required.
quality of joints is completely up to the operator as in machine settings and technique used. (mig welding all not that hard).
there are A lot of things that can go wrong if you do not know what you are doing but if you have the knowledge and equipment it can be safe and productive. i probably missed some obvious things but a small amount of training is definitely required.
If the operator is trained properly safety is most likely his number one priority and will take extra steps to prevent disasters