(This, by the way, is the same scope and mount that Hathcock mounted and used, not only on a M70 but also on a Ma Deuce.) I have a genuine Unertl "USMC Sniper" 8x on a Winchester 52, and while the picture is a tad yellow, this combination has no difficulty knocking spent 12-gauge shells off a target frame at 100 yards. There is no way Unertl or Unertl-type scopes and mounts could be manufactured today for what mass-produced Japanese and Chinese scopes with internal adjustment tubes and, frankly, better optics are sold. The real reason you don't see something like this today, though, is economics. And the sheer length of the scope and girth of the mount made them, well, other than svelte. Some models using fine wire crosshairs werer a tad fragile.
By today's standards, the optics were a bit crude and the light gathering capability was limited. These scopes weren't perfect, but the issues had nothing to do with the external mounts. And, contrary to what you might think, the scope returned to perfect zero every time. Scopes and mounts were made (by hand) by John Unertl and by Lyman. Actually, this design was very popular at one time, both for military precision shooting and for target shooting, both centerfire and rimfire.