A lens is always required to have high quality so that it can produce sharp and accurate photos. Apart from that, the mechanical and optical quality of the lens needs to be considered so that it is durable and can be used for a long period of time.
Sigma as a third party lens sometimes causes doubts as to whether the quality is as good as Canon or Nikon. You can see this video on how to make lenses which are made with great care and perseverance from the workers.