Also to be considered are the ship types and sailing practices.  Viking ships
were designed for open water and the Vikings thought nothing of sailing in all
seasons in the North Atlantic.  One of the least hospitable oceans in the
world.  The Vikings were what are often termed "Blue" water sailors.

The Romans, and most other Mediterranean peoples, used ships designed for
coastal work and seldom went out of sight of land if they could help it.  It was
not uncommon for ships to beach or put into a harbor every night.  The
Roman grain fleets even stayed in harbor during the stormy season.

It is possible that a Roman or other Mediterranean ship was able to make it
across the
Atlantic after being blown off the coast of Africa.  But it is much less likely
than a Viking ship.  And I can't think of a single instance of the Romans
sailing off on voyages of discovery.  The Vikings on the other hand had an
unpleasant habit of showing up just about everywhere.  Unpleasant at least for
the locals.

JH Brothers IV