Modern commercial Newtonian telescopes still use these front coated metal mirrors, although with modern computer design and laser alignment the mirrors, lenses at the eyepieces have become more complex and compound lenses with complex shapes can further correct the aberrations found in the design. Even the humble Refractor has become a high quality optical design now with "APO" Professional quality designs giving excellent quality images.
Mirrors can however suffer from some distortion at the edges called COMA .
Details on that can be found on Wiki
The main benefits of the Newtonian mirror type over a Refrator (lens) type are...
Less distortion reduced chromatic aberration.
Less weight on the telescope mount as heavy lenses are not needed.
Mirror designs have a wider aperture and capture more light for the same magnification.
Shorter design as the light-path is folded as the mirror reflects the image back up the telescope tube.
Cheaper to produce a curved mirror at the objective end than an equivalent objective lens.
Can be manufactured with a wider aperture giving brighter photos for astrophotography.
Can make the mirror in a honeycomb design allowing gigantic optics 80" or more not possible with lenses.
Hope this helps.
Tony